They don’t have to be that tight. You can actually make them too tight with just the zip ties! Tighten them up, then unplug the stepper wires. Now with all the stepper wires unplugged, move the gantry back and forth fairly quickly. If the belts slip, tighten them up a click or two and try again. You’re going to be moving it way faster than it’s ever going to move while cutting/printing, so if it doesn’t slip, you’re golden. Did I mention to make sure the stepper wires are disconnected first? If you don’t remove them, the motors will act like generators and back feed power into the controllers, which will kill them.