Function to redirect the customer to a the checkout page (or a custom page) after adding a product to the cart

redirect_to_checkout.php

<?php
/** 
 * Function to redirect the customer to a the checkout page (or a custom page) after adding a product to the cart
**/

add_filter ('woocommerce_add_to_cart_redirect', 'redirect_to_checkout');
function redirect_to_checkout() {
    return wc_get_checkout_url();
}