There are a lot of different kits out there.
How one does it depends on whether you're looking for form or function. Most of the frame mount kits were designed from the outset to be used on the traditional "double diamond" frame like those used on many (especially old-school) mountain bikes. It would be a relatively simple mating of bike with engine. This is the function side.
On the form side, if you're looking to make a retro-style pleasure vehicle or something that is part show bike, using cruiser style frames can sometimes present some challenges. This is usually due to the graceful sweeping curve of the down-tube, the tubing diameter or both. Sometimes one has to get creative and manufacture a special mount (as I did with my '46 Columbia). However, many kit makers have come up with new mounts or other innovative ways around these problems, so many kits are not that difficult to mount on bikes with curved down-tubes. Don't even ask me about the modern Stingray style frames, as they present their own challenges.
If you're looking for best functionality, look towards a rack-mounted four-stroke engine kit. These can still go on a mountain bike, as long as the bike can accept a suitably sturdy rear carrier. But I have no experience with this type of kit, so I'll leave it to someone else to chime in regarding these.