Fix it with the plate bolted through your bench, or a strong rigid stand of some kind. The bottom of the vice leg needs to go in a hole in the floor, or a block, or have a huge staple to fix it to the bench leg. Lubricate the screw thread well and the casting at the back with the female part of the thread will just sit on there and float in to it's own comfortable position..
The great advantage of a leg vice is that you can hammer things held in the jaws, it's incredibly strong, and the thread doesn't get damaged. Disadvantage is the jaws don't close parallel but you can clamp so hard it's not a problem! Lots of pics on the internet and a few good ones on GJ.