It's safe to say that the person who has managed to adapt some of the rarest lenses in the world to modern DSLRs is probably the one you want teaching you how to do it. We've seen photographer Mathieu Stern slap tiny vintage lenses, 65-year-old plastic lenses, and a Boyer f/6.3 120mm lens from the 1910s onto his Sony A7II, and now we get to see how he does it in the video below:
Finding the right lens adapter, or knowing how to build one yourself, starts with finding your lens' working distance, which Stern demonstrates in the video. (Make sure you set your lens to infinity when you focus.)
He does note that you'll still have to find a way to focus your lens on top of your adapter if it doesn't have a focus ring. Luckily he shows you how to do this using bellows in his first "Weird Lens Challenge" video, in which he adapts that Boyer lens I mentioned earlier. He suggests adding a piece of cardboard between your lens and an m42 macro tube, then screwing that onto an m42 bellows.
Now go out, rip some cool lenses out of old cameras and start making weird (and very, very cool) stuff!
Source: Mathieu Stern