How do I keep the motor from slipping.

From the photos, this looks like a belt-driven motor on an older machine, and a motor that turns on its mount usually comes down to three things. Unplug the machine before you touch anything. First, check the mounting: the motor sits on a bracket with slotted holes and bolts, and if those are loose, the whole motor can twist under load. Snug them, and if there's a slot for adjustment, slide the motor to tension the belt, then tighten everything fully. Second, check the belt. A glazed, cracked or stretched belt can slip on the pulleys and drag the motor around with it, so replace it with one of the same size if it looks worn. Third, check the pulley on the motor shaft. Its set screw or clamp should bite on the flat of the shaft, and if the pulley spins on the shaft, tighten it or add a key. Also make sure the two pulleys line up, since misalignment wears belts quickly. A good belt should deflect about half an inch at the middle of its longest span when you press it, though your machine's manual will have the exact spec if you have it. If the motor still moves after all that, or the shaft has play, take it to a motor repair shop or an electrician before running it again.