How to add a WooCommerce Quick Buy Link without a plugin

functions.php

<?php 
// Outputs the button below the default Add to cart button on Single Product page
add_action('woocommerce_after_add_to_cart_button','hjs_add_quick_buy_link_single', 5);
function hjs_add_quick_buy_link_single() { ?>
	<style>
		.woocommerce div.product form.cart .button.quick-buy {
			background: #f04e3d !important;
    		border-color: #f04e3d !important;
			margin-left: 10px;
		}
		.woocommerce div.product form.cart .button.quick-buy:hover,
		.woocommerce div.product form.cart .button.quick-buy:focus {
			background: #e64135 !important;
    		border-color: #e64135 !important;
		}
	</style>
	<a class="button quick-buy" href=<?php echo site_url('/checkout/?add-to-cart=' . $product_id ); ?>>Quick Buy</a>
<?php }

// Outputs the button below the default Add to cart button on Product Archive / Category pages
add_action( 'woocommerce_after_shop_loop_item', 'hjs_add_quick_buy_link_archive', 15 );
function hjs_add_quick_buy_link_archive() { 
	global $product;
	$product_id = $product->get_id(); 
	?>
	<style>
		.woocommerce ul.products li.product .button.quick-buy {
			background: #f04e3d !important;
    		border-color: #f04e3d !important;
			color: #fff !important;
		}
		.woocommerce ul.products li.product .button.quick-buy:hover,
		.woocommerce ul.products li.product .button.quick-buy:focus {
			background: #e64135 !important;
    		border-color: #e64135 !important;
			color: #fff !important;
		}
	</style>
	<a class="button quick-buy" href=<?php echo site_url('/checkout/?add-to-cart=' . $product->get_id() ); ?>>Quick Buy</a>
<?php }