How to Make a mountain bike into a cruiser?

The frame geometries are so different that short of rewelding the frame you'll never get anything but a fairly crappy cruiser copy. If you insist on trying the things you can do are: 1) switch to a stem with really short reach, you might even want to run it backwards. 2) switch the handle bar to a style known as a North Shore bar. 3) switch seat post to a setback seat post, set it low, get a wide, well padded saddle and slide it all the way back. To get the stance "right" (or maybe less wrong) you could consider swapping the rear wheel for a smaller one, but that would loók right ugly. Another option is to stick a longer fork in there. If you're starting with a 26-inch bike you could swap to rigid fork for a 29er, or a fork for a 622 mm road bike. Do note that: a) there are a couple of different standards out there, so swapping forks can be a real pain if you don't know what you're doing. b) swapping forks is likely to lead to troubles fitting a front rim brake c) swapping forks will increase the load on both fork and headtube and can not be recommended for hard riding or a heavy rider. It will also interfere with handling, but usually not beyond what rider capability. d) IMO this endeavor isn't worth the effort. You'd be better off using ebay, Craigslist or whatever to sell your current bike and buy a cruiser instead.