How to use MarkDown in a custom textarea field?

The best Markdown parser I've found so far is the Parsedown. Something like this:

if ( ! class_exists( 'Parsedown' ) ) {
    require_once( get_template_directory() . '/Parsedown.php' );
}

$parsedown = new Parsedown();
echo $parsedown->text( $content );

If you are dealing with a client this is the way to go, Parsedown is almost bulletproof.

I installed ACF Custom Field WYSIWYG and Markdown Extra.

In functions.php, add code:

remove_filter( 'acf_the_content', 'wpautop' );
remove_filter( 'acf_the_content', 'wptexturize' );
add_filter( 'acf_the_content', 'Markdown' );

Every custom post field that uses acf_the_content will convert to Markdown. You can use the main content too:

add_filter( 'the_content', 'Markdown' );

You can download a copy of PHP Markdown and use it to parse the textarea contents before you save it:

if ( ! class_exists( 'Markdown' ) ) {
    require_once( plugin_dir_path(__FILE__) . '/markdown.php' );
}
$textarea_contents = Markdown::defaultTransform( $textarea_contents );