This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Make sure that your serial ports are connected together, and run the comm program on your PC. Now, for our purposes, lets assume you have the serial card in Slot #2. Force the PC into terminal mode; and, make sure that the PC and II's baud rates are the same-- at least 9600 for a fairly speedy transfer. Make sure the ][ is displaying a 40-column screen. Now, try typing "IN#2" on the ][. Type something on the PC. It should appear on the Apple. (Cool, ain't it?!) In fact, try typing "PR#2". On either computer. Now, you should get an Applesoft prompt on your terminal screen! In fact, you can use the PC comm program as a keyboard for your ][! All right, at this point, you should have data flying back and forth between the two machines. On the ][, type "POKE 33,33". Now, on the PC, tell your comm program to start a text capture. Tell it to add line feeds after carriage returns. On the ][, load the BASIC program, and type LIST. The program listing will appear on the Apple screen and be dumped into the PC comm program's capture buffer. To break communications, type "IN#0" and "PR#0" on the ][. By: Brian Hammack