Since I decided to do just one side and the back I measured the back first. I wanted the background to come to within 1 inch of the side I was not going to cover, this was because I wanted to taper the edge off and not have a sharp square end showing. I then measure the left side of the aquarium, I decided on this side I was only going to cover about 3/4 of it. Then cut your foam to fit. My 75 gal aquarium was 4 foot long on the back and I needed about 10 inches on the side. I measured 1 inch shorter because of the taper effect or the back piece. I also had to make my background in 2 pieces to get it inside and past the center support when I am finished. I also cut the pieces 2 inches shorter so that there will be sand under the edge when I am done. I started with the back corner. I first used the silicone and glued the two corner pieces together along with the first bottom "rock" cut out of the foam. I used the finishing nails to hold it together until it dried. Some people cut the rocks first and test stack them. I just cut them out of the foam as I needed them. I just used my imagination on the shape of the rocks. This is where a foam cutter comes in handy. If you dont want to make one or have one, just use an electric knife or even just a normal knife and cut and carve. Remember rocks are not perfect but also remember small details will not show up after you coat the thing in cement.