How do I use foil to keep heat in cage?

[tubeyes]

Hello all! New BP owner, just getting everything set up and finalized for his cage.

I was wondering if there was a special way to use foil when keeping heat in the cage, or do you just coat the bottom and 3 sides of the cage with it? I've read a little about people doing it, but I couldn't seem to find a proper guide to the best/most recommended way of foiling.

Anyone have any suggestions, pointers or redirects?

Thanks!

edit:

I'm using a UTH and the equivalent of a 10 g plastic tub.