I like to create a point of sale system.  What is the best way to do credit card processing?  Thanks.

Pick your preferred credit card processing gateway (based on overall terms they offer, what cards they cover,...) then look at what integration APIs they offer and then see how you can use those APIs.

There are some products out there that already do this, google on 'filemaker plastic' for instance...

But if you are really on FM 11 still then you will have a hard time with this, that's very old.

Wim, any experience with Square/FM integration? Is it possible? I'd like to support ApplePay, smart cards, etc.

No personal experience but they have a REST API so: yes.  Should be feasible.  Easier in 16 no doubt, in 15 you'd have to use a plugin for the REST communication.  I tend to use the free BaseElements plugin.

https://docs.connect.squareup.com/api/connect/v2/

Yep, that's what I hope. Just wondering if you brainiacs at Soliant have done this already.