The print is put under UV light until the exposed sensitizer turns from yellow-green to 'confederate grey'. The sun or a UV tanning lamp can be used as a UV source. See the The Big Cyanotype Exposure Survey
for more information on exposure times and techniques.What I did:
The frame with paper & negative was placed directly under a home tanning lap set face-down on a table. This is exactly the same setup I use to make PCBs, albeit with positives and photosensitive copper-clad fiberglass board.
I did a number of exposure tests. First I tried recommendations from the survey, 15 minutes and 10 minutes. These were way over exposed. Next I tried 5, 3, 2.5, 2, 1.28 (the exact time I use for PCBs), and 1 minute. I settled on 1 minute and 45 seconds.