To mount the vise on your bench, drill a 3/4" hole through the bench top or bench leg. Place the vise through the hole and on the underside of the bench (or opposite side of the bench leg) thread the large flange onto the vise screw.
There is a small hole in the flange for a pin or screw to keep the threaded flange from spinning while tightening the vise in position. Using the flange as a guide, drill a 3/16" hole in the bench deep enough to accept most of the pin. The pin should protrude enough to fill the hole in the vise flange.
Prevent the vise from moving while in use by sandwiching a piece of screen between the vise's wooden block and your bench. You can find a sheet of drywall sanding screen from your local hardware store in the coarsest grit available. Cut it to size and a 3/4" hole in the center for the vise screw.
Over time the vise gripping ball can compress under constant clamping pressure. If this happens the long shaft of the vise will get harder to rotate and harder to slide within the ball. The cure for this is to spring the ball open a little. To do this place two good sized straight-slot screw drivers in the open cut-side of the ball and give a gentle pry. Once this is done the long shaft should slide freely again.