How to Create Frontend Plugin in TYPO3?

# Create /Configuration/TCA/Overrides/tt_content.php

<?php
if (!defined('TYPO3_MODE')) { die('Access denied.'); }

call_user_func(function () {

	# Define extension key
	$_EXTKEY = 'extension_key';

	# Should move code from ext_tables.php to here > To register plugin
	\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
	    $_EXTKEY,
	    'Pi1',
	    'Title'
	);

	# Prepare plugin's signature
	$extensionName = strtolower(\TYPO3\CMS\Core\Utility\GeneralUtility::underscoredToUpperCamelCase($_EXTKEY)); 
	$pluginName = strtolower('Pi1'); 
	$pluginSignature = $extensionName.'_'.$pluginName; 

	# Add list_type to tt_content
	$GLOBALS['TCA']['tt_content']['types']['list']['subtypes_addlist'][$pluginSignature] = 'pi_flexform';

	# Add Flexform
	\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue(
		$pluginSignature, 
		'FILE:EXT:'.$_EXTKEY . '/Configuration/FlexForms/pi1_name.xml'
	);
});

<!-- /Configuration/FlexForms/pi1_name.xml -->

<T3DataStructure>
    <sheets>
        <sDEF>
            <ROOT>
                <TCEforms>
                    <sheetTitle>Fields</sheetTitle>
                </TCEforms>
                <type>array</type>
                <el>
                    <headline>
                        <TCEforms>
                            <label>Headline</label>
                            <config>
                                <type>input</type>
                                <eval>required</eval>
                            </config>
                        </TCEforms>
                    </headline>
                </el>
            </ROOT>
        </sDEF>
    </sheets>
</T3DataStructure>