It depends on which model of Mk VI to which you're referring. However, there are generally two methods people tend to use:
The first is the magnetic approach - in one half of the torso you'll have some high-strength neodymium magnets, and in the other, something for the magnets to attract to - a metal plate, or other magnets. Neodymium magnets are cheap and easily-acquired, but there's the danger of your torso coming detatched while you're moving around since there's no mechanical fixture.
The second is the mechanical approach - in one half of your torso you'll have a male quick-release buckle/clip, and in the other, a corresponding female quick-release buckle/clip. Again, these fittings are easily-acquired and very cheap, but may prove more difficult to unclip.
I made my suit out of foam, so its flexible. I basically just attached the two top parts, that go over your shoulders, together. I widened the neck hole just enough to where my head can fit through, and I put the chest on like a shirt. The bottom is secured together with Velcro and snaps. I couldn't afford magnets so that's what I did.