You are building, or helping build, an online shop that is not WooCommerce (a custom stack, a headless frontend, a different framework etc.), and you want to accept payments through Woosa Payments.
If your shop runs on WooCommerce, you do not need any of this. There is a ready-made plugin. Install it, configure it, done. This guide only matters for shops that are not WooCommerce.
You do not need to be a payments expert to follow this. Wherever a technical term shows up, it is explained in plain words the first time, and there is a glossary at the end. You will need someone who can write a bit of backend code to actually wire it up, but this guide is meant to make the whole journey understandable before a single line is written.