I'm going to bleach a logo onto a t-shirt...?

So I found this DIY thing that shows you how to put a your own logo onto a dark colored t-shirt by laying letters on it and spraying the shirt with a bleach and water mixture. I have pre-cut letters that I'm using which are made from a sort of card stock type paper. Will the bleach soak through the letters? Should I put plastic wrap on them or something? Also do you think the discoloration will soak through from around the letters and mess up their shape? I know this would be a sort of trial and error thing but I only have so many letters and I don't want to waste them.