This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
For a IIgs, you connect to the IIgs RGB monitor. For other Apple II's, you use a video quality 'RCA cable' to connect to a composite color monitor, such as one of the Apple II monitors or a third party monitor like the Amdek Color- 1. (The IIgs has a composite color output which can go to a composite monitor; but, for IIgs super-res, the results are unsatisfactory.) An alternative to a composite monitor is to connect to a color TV directly or through a VCR. Most VCRs and many modern televisions have a composite video input port. Rather than buying a new monitor, a person can usually use an RCA cable to connect the Apple to a spare VCR or television. The stabilization logic built into even a cheap VCR can go a long way toward cleaning up an ugly picture.