How to pre-populate WordPress editor with default content

This code has to be pasted into your theme’s functions.php file. It can be customized to deal with custom post types, as shown on line 16 and after.

add_filter( 'default_content', 'pu_default_editor_content' );

function pu_default_editor_content( $content ) {
    global $post_type;

    switch( $post_type ) 
    {
        case 'post':
            $content = 'Default content for blog posts.';
        break;

        case 'page':
            $content = 'Default content for pages.';
        break;

        case 'portfolio':
            $content = 'Default content for your portfolio pages.';
        break;

        case 'products':
            $content = 'Default content for blog posts.';
        break;
    }

    return $content;
}

Credit: Paulund