Use a drill bit and drill a larger hole in the top, do that down the PVC. Use a smaller bit to drill through the large hole to start another one at the bottom into your board (This hole is to prevent cracking in the pipe when putting the screw in. Not completely necessary but it sucks when the pipe racks around the screw). Then using an extension drill the screw into the pipe into the board.
The problem with mounting the PVC from bottom upwards is it is a lot more weak and can crack as mentioned before. It is easier but not as efficient.