At photo.net I posted a answer about using a laser scope to do this, which I have found to be very easy.

http://www.photo.net/bboard/q-and-a-...?msg_id=0056Bo

The reason I like the laser over the projected negative is that you can adjust both the lens and the negative carrier to the baseboard separately. If both are out of alignment you will have a lot of difficulty adjsuting using a projected image - I know I did.