You can also get cables wired as null modems. These work just as well, but you can't also use them with real modems. My ST and my laptop both have standard DB-25 sockets, so I can use a null modem connector with a normal modem cable. Other laptops might need custom cables. Any Atari telecommunications software should work.
Most laptops already have adequate file transfer programs built-in.
Turn on the telecommunications software of both computers. If one computer won't accept some of these parameters, set the other one as shown above and pray for luck. If you're unlucky, work your way through all the possibilities until you hit on one that succeeds. Make sure the carriage returns come through correctly too. If your modem software supports macros, save your successful configuration for future use.