How do I cover a square cake with fondant and eliminate the seems???

I have to do normal squares and then I have a 8" square which will be 12" tall...help!

In that case, you wouldn't want to cover it. You would want to apply it in strips. One strip for each side, and one small square for the top....oh, I think you already know that, rereading your post. Ok, gently rub them together, and you might have to put on a fondant snake to hide the seams. It could look cool.