WooCommerce: Filter a product shipping weight

wc-filter-product-shipping-weight

// Filters both variations and simple products with the same function
add_filter( 'woocommerce_product_variation_get_weight', 'filter_product_weight', 10, 2 );
add_filter( 'woocommerce_product_get_weight', 'filter_product_weight', 10, 2 );
/**
 * @param float      $weight
 * @param WC_Product $product
 *
 * @return int
 */
function filter_product_weight( $weight, $product ) {

	// 1. Check if the product has a set weight already. May be you want to add to the weight
	if ( 0 < $weight ) {
		$weight += 100;
	}

	// 2. Assuming you did add a setting in your product edit
	// screen so the admin can change the additional weight you will add, check the setting and add the weight
	if ( 0 < $product->get_meta( 'additional_weight', true ) ) {
		$weight += $product->get_meta( 'additional_weight', true );
	}

	return $weight;
}

Where do I put this?