First I tested it on a Tiki 17 and it was working ok. (Paypal real payment/ Sandbox didn't work).
I agree the doc page is not very documented and example is missing. (another sticky on the wall )
Explaining everything from A to Z is not an option (I need to set it to explain it).
I suggest you to check the doc for cart and payment too. Sometimes things became clearer when looking at a related feature.
But if you are stuck on a specific part and can’t wait for the doc to be updated ask here or private.
You can also use one of the https://tiki.org/Consultants" class="wiki wikinew text-danger tips">Tiki Consultants if you want more support and things done. ;)