functions.php

<?php

/**
 * Add to cart redirect to checkout.
 * 
 * @param string $url
 * @return string
 */
function woocode_wc_add_to_cart_redirect_to_checkout( $url ) {
	return wc_get_checkout_url();
}

add_filter( 'woocommerce_add_to_cart_redirect', 'woocode_wc_add_to_cart_redirect_to_checkout' );