How to cross bra straps over?!?

Basically, my friend has small breasts and she gets really down and self conscience about it. I've tried telling her not to get down about it and having boobs isn't everything. Boob size doesn't mater and all that, but that only seems to make her worse. So i looked up on the internet for her on how to make breasts appear bigger. We've tried most of the ideas we found and she still wasn't happy with the results.. One idea we found was to cross bra straps over and not meaning to sound idiotic, but neither ogre us knew how to do it.. So how'd you cross bra straps over?