To install the PayTomorrow WooCommerce plugin all you have to do is install it from the plugins section in your WordPress' backoffice. Just look for PayTomorrow and we should be the first result. If not here’s the link: https://wordpress.org/plugins/paytomorrow/
Once installed, head over to WooCommerce -> Payments -> PayTomorrow -> Manage and set your new PayTomorrow credentials. Here’s a couple of screenshots of what it looks like.