Use this info in addition to that already mentioned...
When you take the bolts out, place them in the correct hole in the new pump on a bench as you pull them, one by one. After you remove the old pump, place it one the bench next to the new one and move the bolts over one at a time to the same location on the old one. This way you will maintain a reference to where the bolts go when you put it back together.
Clean the mating surface thoroughly as mentioned. I like to use a drill with a small wire wheel.
Run a tiny bead of RTV on each side of the new gasket, but not so much that it will squeeze into the water channels upon assembly. Just a dab will do it.