There's also a trick for getting the second condenser glass into place without covering it with fingerprints, chipping/scratching it against the lower glass, or worst of all dropping it and doing heaven only knows what damage. Make a loop of thread, fold it in half, grab the fold, and put the two looped ends around the condenser so they suspend it, convex side down. If you made the loop short enough (or if you "choke up) a little on the fold), the thread will hold the glass securely while you lower it into place on top of the corrugated band, and then you can let the tension off the loop and work the thread out from under the glass by pulling on one strand at a time.

Any other method is very likely to result in fingerprints, at best, and much worse (more expensive) problems as a strong possibility. Replacement glass for those costs a good chunk if you have to buy it from Classic Enlargers...