Hi there,
1. Templates files can be found within the /wp-content/plugins/woocommerce/templates/checkout directory.
2. Copy the needed file into the directory within your child theme named woocommerce (create if it does not exist), keeping the same file structure but removing the templates subdirectory
3. For example, if you need to overwrite file /wp-content/plugins/woocommerce/templates/checkout/form-checkout.php, copy this file to /themes/gecko-child/woocommerce/checkout folder
Foster