You mean how to physically arrange them in your tank space, right?
Larger pieces should go on the bottom. Smaller pieces on top. (This is probably obvious)
You can either stack the rock up against the back glass, or make a stack in the middle of your tank. There are reasons why you might want to do either.
Stacking in the back is good because: added support (rocks less likely to topple), more room for the fish to swim towards the front of the tank.
Stacking in the middle is good because: it is less likely that a fish would get stuck in the rockwork with this setup, you will get the most light in the middle of the tank (good for corals and life on the rock), the water will circulate all around your rocks (good for both the rocks and the fish in the tank), and the fish will swim around the rocks in the middle (probably closer to a natural situation for the fish, as they swim around rocks in their natural habitat).