Well we can't start sticking them on the rubik's cube if we don't know if they will fit nicely now can we?
Take your rubik's cube and try the pieces on one side of it. If you cut them out properly they will fit perfectly like mine do.
Now you have a choice. Will you remove the original stickers or just put the new ones over the olds?
If you want to remove them just take your knife and try to remove the original stickers without digging into the cube.
I really didn't care about the old stickers so I just left them on. (I'm kinda lazy and I didn't want to damage my rubik's cube) Besides the double sided tape will stick to the stickers as good as it would to the rubik's cube.