At the most basic level it's simple:

From the store admin:
- create one category
- add one product to it
- set it up as a variable-price product as described in this article: http://www.zen-cart.com/content.php?...ct-or-Donation
- add and configure whatever payment module code you need for your gateway
From the storefront:
- test checkout and payment from beginning to end

- then once you're satisfied that it can do the commerce requirement, work on styling the site to suit your needs