First you need to make sure the Internal diameter is what the tap size you are using calls out for. You can find the major/minor diameter in a Machinist handbook, or probably on the internet. Depending how accurate you want your tapped hole to be, indicate your peice in(4 jaw). Your best bet would have as little stock sticking out. Lowest RPM in low range. Lube the heck out of the tap and inside the tube and continue when its tapping. With your tailstock free, gently feed the tap in. If the tap catches and spins, kill the lathe, tighten the tap up, relube and start it up again. You can reverse the chuck and back the tap out to free any chips, but I wouldnt suggest taking the whole tap out because it can be a pain restarting it. When you are backing out, help the tap by gently pulling the tailsock back. Again, dont have the tailstock locked down, it should be free moving on the ways. I like to put a little chamfer on holes I am tapping. It gives it a nicer appearance and tends to be a little easier to start because your protecting the first initial thread from getting banged up.(do the chamfer before tapping)
Very simple. once you get bored with it, try internal threading!