just put it on the back outside of the tank useing bluetak (sp) or celotape,and use a much as you want till it looks ok and tidy and then cut any bits that poke out the side if you know what i mean.
You can get those that fit to the inside of the tank, they are usually a bit more expensive and require fixing with a silicone glue (like bathroom sealant).
Personally I dont like the backgrounds. I have used plastic plants that look as real as possible to create a backdrop.
i have used vaseline to press the black backing on to my large tank. it makes it like applying a decal. its quite tricky to get all the bubbles out but it does look better than with sellotape.
i have a 5G that has a mirror on the back that i am going to seal a black background on the inside of the tank to cover the mirror so i can have a betta in it