I'm attempting to adapt the masking code from lesson 20 to my own game engine, but I can't seem to get it to work. Does masking only work if there's nothing underneath the image but its mask, or am I the one causing the problem? Based on everything I know (which is not a lot), it seems like it's me. I'll post some code in the morning, but I just wanted to post this now and see if there are any common masking issues that I should be aware of. Finally, should I be using alpha testing instead of masking to draw sprites for a game? I'm used to D3D, so I'm unsure about the correct approach in OpenGL.

The correct approach is not to use masking at all.
Instead use either alpha or alphatesting.

The masking method described in lesson 20 has only a few limited uses and if you don't know exactly what those are then don't use masking.