How do I add credit card payment to my website?

I have already created the checkout page on my website using html, css, javascript and php. Now I only need to make it actually work. I have seen tons of code snippets online and I was overwhelmed and confused. For example I wanted to add KNET payment to my website and I found this: https://docs.checkout.com/payment-methods/cards/knet

I have no idea how to even start adding this to my website. It shows snippes of json objects but how do I add that and make it online payment? My website is complete and the only thing left is adding the online payment. Any help is greatly appreciated! Thanks :D