My method for using a 12" Makita beam planer:
1. If you're bound and determined to buy one of these things, then also hire someone else to run it. If you decide to use it yourself then shop around for a good chiropractor -- you'll be seeing this guy a lot.
2. Put the timber on sawhorses -- shorter to mess up your lower back, higher to screw up your shoulders. Or use short in the morning and taller in the afternoon to spread the agony around.
3. Flip the timber and find the best face, take a couple 1/32" passes off. As TG said, more then that and you'll plug the planer up solid.
4. If the timber has a taper, now is the time to get the 16" saw out, find the best edge and rip the other edges parallel. Of course a 16" saw won't rip all the way through an 8" stick, so I finish the cut with a 3 1/2" planer. Sometimes this even squares up the timber. Sometimes.
5. So you've got the timber planed on the top, now square the left face to the top face. As TG mentioned you can apply a bit of 'english' here. The planer is heavy enough that if you have it way over to one side it will cut more on that side then the other. Take off a pass or two, check for square, repeat. Gradually you'll work the planer back to the center - your last pass will be right down the center of the face.
6. Flip the timber and plane the right face, same method. With three adjacent sides square - flip the timber to the backside and plane the last face, same method.
7. Volia, you've got a square timber ready for mill rule and your chriopractor is going to buy a really nice boat.

To be serious -- The Makita beam planer is a great tool. With sharp blades it gives an outstanding clean finish that is better then sanding. Just don't underestimate what lugging around a fifty pound boat anchor all day, day after day - will do to your body.

Cheers;