Cutting holes in plexiglass needs to be done in a proper fashion or chipping and cracking can occur. You can drill easily into plexiglass using a normal drill and metal cutting drill bit. You will need to take your time, and have a steady hand to achieve the desired results.
Measure and mark where you want to drill your hole.
Place the plexiglass on a piece of wood that you don't mind drilling into. It is best to use a hard surface rather then a soft surface such as foam to prevent the plexiglass from bowing and cracking while you drill into it. Use your spring clamps to hold the plexiglass in place so that it will not move while drilling.
Apply the masking or painter tape over the area you are going to drill. This tape will prevent chipping in the area you are drilling. As a preventative measure, drill a small pilot hole; for example, if you are drilling 1/2 inch hole start a pilot hole of a 1/8 inch first and gradually increase from that to a 1/4 then to a 1/2.
Start your drill and gently push on where you need to drill. Keep a steady hand and do not push too hard. Be slow as it works it's way through the plexiglass. Use water in a spray bottle as a lubricating liquid to keep the plexiglass cool, spray on the plexiglass as you are drilling to keep it wet. Once through the plexiglass with your drill, you are done.