How can we integrate a third party payment gateway into an ecommerce store?

[DanielHighETH]

Hello, we are currently on a hackaton and we would like to integrate our payment wall into a shopify (for example)

When a potenional customer wants to check out, they select our payment method on the store, it will redirect them to our 3rd party website.

I've created a partners account and we have a dev store.

We are unsure what to do now, do we create an App, an extension or what is our next step? 

What would be the best practice here, we just want to test it - so it wont be a real app, which must be approved etc. 

Thanks a lot