Thoroughly wet the back glass by either spraying clean water or wiping it with a soaking sponge/cloth. Having more water on the surface makes the installation much easier and bubble-free. Make sure you put a rag under to catch dripping water so it does not damage your furniture/ aquarium stand.
Trim- off the excess background film. You should be trimming off only two sides if you aligned factory-cut straight corners properly. The bottom part will be covered by gravel so don't worry if it is not 100% straight. For the vertical side, easiest and safest way is to hold the razor 45 degrees to the edge of your tank and rest the side of your razor on the edge of your aquarium as you go cut down. Be extra careful not to cut the silicone of your aquarium. This may cause your aquarium to burst.