When adding a form using Form API, best practice is to use dependency injection when possible. Admin form classes typically 'extend ConfigFormBase'. If you want to add to it, it'll look like this.

<?php

namespace Drupal\my_module\Form;

use Drupal\Core\Config\Config;
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Messenger\MessengerInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Class myCustomAdminForm.
 *
 * @package Drupal\my_module\Form
 */
class myCustomAdminForm extends ConfigFormBase {
  
  /**
   * @var $configFactory
   */
  protected $configFactory;
  
  /**
   * @var Config $myConfig
   */
  protected $myConfig;

  /**
   * The Messenger service.
   *
   * @var \Drupal\Core\Messenger\MessengerInterface
   */
  protected $messenger;
  
  public function __construct(
    ConfigFactoryInterface $configFactory,
    MessengerInterface $messenger
  ) {
      parent::__construct($configFactory);
      $this->myConfig = $this->config('my_config.settings.my_settings');
      $this->messenger = $messenger;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('config.factory'),
      $container->get('messenger')
    );
  }
  
    /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'my_config.settings.my_settings',
    ];
  }
  
  /**
   * This needs to return a string that is the unique ID of your form. Namespace the form ID based on your module's name.
   */
  public function getFormId() {
    return 'my_module_admin_form';
  }
  
  /**
   * This returns a Form API array that defines each of the elements your form is composed of.
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
  
  	$form['text_input'] = [
  		'#type' => 'textfield',
  		'#description' => t('Enter data into this field'),
  		'#default_value' => $this->myConfig->get('my_config_values')
  	]
  	
  	return parent::buildForm($form, $form_state);
  
  }
	
  /**
   * Validate the form
   */  
  public function validateForm(array &$form, FormStateInterface $form_state) {
  	$values = $form_state->getValues();
  	if(empty($values['text_input']) {
  		$form_state->setErrorByName('text_input', t('You must enter data into this field.'));
  	}

  }
  
  /**
   * Submit the form
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
  	$values = $form_state->getValues();
  	
  	$this->myConfig->set('my_config_values', $values['text_input']);
  	
  	$this->myConfig->save();
  	
  	$this->messenger->addStatus('Your value has been saved.');
  	
  	parent::submitForm($form, $form_state);
  }

}

Trying to debug twig? Not rendering? Try this from within your twig file...

<script>console.log({{ _context | json_encode | raw}});</script>

Also includes dependency injection

MY_MODULE.services.yml

services:
  my_module.cool_service:
    class: Drupal\my_module\Services\CoolService
    arguments: ['@logger.factory']

/src/Services/CoolService.php

<?php

namespace Drupal\my_module\Services;

use Drupal\Core\Logger\LoggerChannelFactoryInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * CoolService returns what you give it in an array.
 */
class CoolService {


  /**
   * Logger Factory.
   *
   * @var \Drupal\Core\Logger\LoggerChannelFactoryInterface
   */
  protected $loggerFactory;


  /**
   * Constructs the Cool service.
   *
   * @param \Drupal\Core\Logger\LoggerChannelFactoryInterface $loggerFactory
   *   A Guzzle client object.
   */
  public function __construct(LoggerChannelFactoryInterface $loggerFactory) {
    $this->loggerFactory = $loggerFactory->get('my_module');
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('logger.factory')
    );
  }

  public function doSomething($someData) {
    $output = [
      'my_data' => $someData;
    ];

    $this->loggerFactory->error('Lets log a message!');
    
    return $output;
  }


}

Calling the service

$coolService = \Drupal::service('my_module.cool_service');

$returnedData = $coolService->doSomething('My Data');

echo $returnedData;

Need to send an email from your custom module?

First, let's make a template file so you can add some HTML markup if you want. This will go in your custom_module/templates directory.

<h1>Here is a header for your e-mail</h1>
{{ text|raw }}

Sincerely,
{{ sender_name|raw }}

Some pieces that will go into your custom module

Include the template in your hook_theme()

Be sure to clear your cache for this template to be used.

/**
 * Implements hook_theme().
 * To add the template definition.
 **/
function MY_MODULE_theme($existing, $type, $theme, $path) {
  return [
    'my-custom-email' => [
      'variables' => [
        'text' => NULL,
        'sender_name' => NULL,
      ],
    ], 
  ];  
}

Add hook_mail()

/**
* Implements hook_mail().
*/
function MY_MODULE_mail($key, &$message, $params) {
 $options = array(
   'langcode' => $message['langcode'],
 );
 switch ($key) {
   case 'my_custom_key':
      $message['headers'] = $params['headers'];
      $message['from'] = $params['from'];
      $message['subject'] = $params['subject'];
      $message['body'][] = $params['message'];
      $message['params']['files'] = $params['attachments'];
      break;
 }
}

Now, you can wrap the emailing into a function while using variables such as the recipient, sender, etc.

<?php
  
function _send_my_custom_email($data) {
  // Set a reply-to value
  $reply_to = $data['reply_to'];
  
  // Set a from value
  $from = $data['from'];
  
  // Set a subject
  $subject = $data['subject']
  
  // Set a sender name
  $sender_name = $data['sender_name'];
  
  // Set a recipient email
  $to = $data['to'];
  
  // Set your message body
  $body = $data['body'];
  
  // Use the theme to send the message
  $theme_body = array(
    '#theme' => 'my-custom-email',
    '#text' => $body,
    '#sender_name' => $sender_name,
  );
  
  // Set up the Drupal Mail manager
  $mailManager = \Drupal::service('plugin.manager.mail');
  
  // Tell the mail system what module you are sending this from
  $module = 'my_custom_module';
  
  // Set up the parameters for your email from the variables
  $params['message'] = drupal_render($theme_body);
  // Set some header information
  $params['headers'] = [
    'content-type' => 'text/html',
    'MIME-Version' => '1.0',
    'reply-to' => $data['from'],
    'from' => 'sender name <' . $from . '>',
    'Return-Path' => 'sender name <' . $from . '>',
  ];
  $params['from'] = $from;
  $params['subject'] = $subject;
  
  // Need to attach a file? Maybe uploaded through a form
  if(!empty($data['attach'])) {
    $file_info = array();
    $file_info['filepath'] = $data['attach']['uri']; // File path
    $file_info['filename'] = $data['attach']['filename']; //File name
    $file_info['filemime'] = $data['attach']['filemime']; //File mime type
    $params['attachments'][] = $file_info;
  }
  
  
  // Set a key so you can send different types of message
  $key = 'my_custom_key';
  
  // Get the language code from your site
  $langcode = \Drupal::currentUser()->getPreferredLangcode();
  
  // Set sending to true in case there is other logic that needs to happen before
  $send = true;
  
  // Send the E-Mail
  $result = $mailManager->mail($module, $key, $to, $langcode, $params, NULL, $send);

}

Now, you can call the function to email while passing along some parameters

// Within a function that you are using that requires an email to be sent, you call the function with some data

use Drupal\file\Entity\File;

function _something_should_be_happening() {
  if($something == 'happened') {
    $data['reply_to'] = 'reply_to@example.com';
    $data['from'] = 'from@example.com';
    $data['subject'] = 'Subject text here';
    $data['sender_name'] = 'John Doe'; 
    $data['to'] = 'receiver@example.com';
    $data['body'] = 'A long value that can also hold <div>HTML Markup</div>';
    
    if($add_a_file == TRUE) {
      // Need to send a file attachment?
      // Load a file
      $file = File::load('FILE_ID');
      // attach some file info
      $data['attach']['uri'] = $file->getFileUri();
      $data['attach']['filename'] = $file->getFileName();
      $data['attach']['filemime'] = $file->getMimeType();
    }
  
    _send_my_custom_email($data);
    return;
  } else {
    return;
  }
}

Often times when using the Drupal Form API creating custom forms, you'll need to get a select list of Taxonomy terms as options. These snippets will help you look up terms in a vocabulary and put them in your options array.

Drupal 8

// Create empty options array
$options = array();

// Taxonomy vocabulary machine name
$taxonomy = 'tax_machine_name';

// Get all the taxonomy terms from the vocabulary
$tax_items = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadTree($taxonomy);

// Inject into the options array
foreach($tax_items as $tax_item) {
  $options[$tax_item->tid] = $tax_item->name;
}

// Create a select form element
$form['taxonomy_options'] = array(
    '#type' => 'select',
    '#options' => $options,
    '#required' => TRUE,
);

Drupal 7

// Create empty options array
$options = array();

// Taxonomy vocabulary machine name
$taxonomy = 'tax_machine_name';

// Get all the taxonomy terms from the vocabulary
$vocab = taxonomy_vocabulary_machine_name_load('tax_machine_name')->vid;
$tax_items = taxonomy_get_tree($vocab);

// Inject into the options array
foreach($tax_items as $tax_item) {
  $options[$tax_item->tid] = $tax_item->name;
}

// Create a select form element
$form['taxonomy_options'] = array(
    '#type' => 'select',
    '#options' => $options,
    '#required' => TRUE,
);

Creating a node programmatically in Drupal 8? There is much more to it that this, but this is the start...

use \Drupal\node\Entity\Node;

$node = Node::create(['type' => 'CONTENT_TYPE']);
$node->set('title', 'YOUR TITLE');
// Simple textfield
$node->set('field_TEXT_FIELD', 'Your Text Here');
        
$node->save();

In Drupal 7, you might want to output a simple class when using the l() function to create a link

l('Link Text', 'path', array('attributes' => array('class' => 'custom-class')));