For this prank, you'll need some pongy cheese. That's it!
Pinch the soap from the bathroom. You'll be replacing it with something much more fragrant shortly...
Get someone to cut the cheese (ha-ha! Cut the cheese!) to a shape like a bar of soap. It helps if the cheese looks like soap as well.
Put your 'cheesy soap' in the bathroom.
Now your victim's lovely soap will make them smell of cheese! What a sweet prank! So right it's pong... er, wrong!