I don't have any foreign clients, but I do know someone who literally wrote the book:
1040NR? or 1040? U.S. Income Tax Returns for... by Jean Mammen, EA
It's available on Amazon and all your favorite places, so you could have it tomorrow!
I think the 7th Edition is the latest version, so make sure you get at least the 7th Edition.
Also, if your issue is NOT how to prepare the return but more how to prepare the return in ATX for e-filing, you may need to call tech support.
Hopefully, someone with international clients will pop on here today to help you.