Depends on who you ask, really!
For me, it was very easy. I took the stickers, the keyboard, gave it to my gf and asked her to align them on the board. Easy.
Her part, however, not sure
I'd say you should just put them on softly without pressing, align them with toothpicks, then press them. Afterwards, your fingerprints and some glue residue will be seen on the sticker. I've tried removing the residue with some microfiber cloth, but the fibers stuck on it. I'm going to try with a sunglasses cloth or something like that - something without fibers.