It's not massively difficult to get some initial results - on the left hand side of the viewfinder housing on the 5d mkII is a white mark that looks a bit like a london tube sign (circle with a bar through it). This is the focal plane mark - it's where the sensor is. Knowing that, you then need to mount the other lens/camera body so that where it expects the film to be is aligned to the focal plane mark (on a folding camera, that could be by leaving the back open and not fully extending the bellows so that the sensor mark is the same distance from the lens as the film would be with the camera fully opened, whereas with an old lens you'd need to know the registration distance). Then, if it's a camera with an integrated shutter, open it onbulb mode and get the right exposure using iso and speed on the canon. Biggest difficulty will be mounting the lens/old body so that it's reasonably light tight and doesn't block the canon controls.
Note that I said it's easy to get some results - you'll need to spend a bit of time on working out the best mount to get
good results