Welcome to the forum Contour!!!!
The best method that I have found for applying backgrounds is to use Petroleum jelly. In fact, as soon as I find time, I am working on an article or something along those lines on how to go about things.
All I do is spread the petroleum jelly on the back of the tank...trying to keep it on an even, thin layer...and then simply stick the background to the tank.

After you have it on the tank, be sure to trim the background so that it is exactly the size of the tank

The last step is to smooth out the background in order to get any air bubbles and/or extra petroleum jelly out. You can use anything from a credit card to a rolling pin.

That is really all it takes and it lasts for a VERY long time. Also, dont worry about any small streaks of petroleum jelly...it will become more and more clear over time.