I agree with the two above, if it's a sink tube, always ALWAYS somehow make a starter hole, whether it's with a nail or whatever. My first saber ended up having a dowel in the bottom to attach a sink knob so that was a little sturdier, drilling through the two. Though, the top of the saber had many little holes with nothing but the sink tube. Go slow, in spurts and each time make sure the tube is still in shape.
If you're drilling something flat, like the bottom of a graflex tube, Drilling dents the brass inward. Put a thick screwdriver handle down in the tube, while standing it up and rubber mallet smash the screwdriver into the bottom, sandwiching it with the table top. Comes back flat as ever.
If you need to make a larger hole with just a hand drill you can pick the size drill bit one smaller than what you need and slowly go in circles to widen the hole. Like the drilling above, check each pass. Taking the time will save you a lot of frustration.
A simple tabletop clamp is the best. I usually wrap whatever I'm working on in a dishtowel to protect it. Masking tape or electrical tape will work as well!
Hope I helped!