How can I drill to an exact depth with a hand drill?
Homemade Stops
Wrap a bit of tape around the bit, to mark the desired depth.  Then carefully drill your holes, stopping when you've reached the tape.

I've also seen this done with a bit of wire.

WARNING: Wrapping the wire in the wrong direction, can lead to property damage, injury, and death.
Store Bought Stops
Or you could purchase a drill stop.

Which are also available in plastic

Portable Drilling Guide
There are also portable drilling guides available.

I tried one of these and it was total crap, so I wouldn't recommend it.  Your mileage may vary though, so give it a try if you like.