Create a custom payment that uses a transaction payment gateway by CorvusPay

[RUSHWAYC]

Hi!

So, I need to install CorvusPay as a custom payment gateway on my shop because all the ones supported by Shopify are hella expensive. So I got this documentation here - https://cps.corvus.hr/public/corvuspay/CorvusPay_IntegrationManual.pdf

I need to create a POST method, but I have no idea where to do that. Should that be done as an action="" on the <form> in the checkout liquid? What is the name of the checkout liquid on the Dawn theme?

I also need to get the following data: order_number, amount, and cart (cart description). But I don't know what generates that data and if the fields should look like: <input type="hidden" name="order_number" value="1233">

I tried to get in contact with Shopify programmers but $4000 for this is an overkill in my opinion. Especially as at one store I know of they set up CorvusPay without any programming knowledge and here I am sitting like an idiot with some programming experience and having no clue what to do.