This is a super easy one to spam, but it does take a while to actually get it finished. You will need to use a spell called “Muffle”. This is a good one, since you can cast it while sneaking and it will help you out rather than alert enemies to your location, you can pretty much spend the entire duration of the game, spamming the hell out of this spell in order to level up your illusion ability much faster.
Another way to quickly increase your illusion magic skill is to use the “Courage” spell. You cast this spell on NPC characters in the game. It should allow you to further increase your illusion abilities and it should be faster than using the muffle method, although it is a lot slower. You can spam muffle as you go about your daily chores, but at least this is a useful alternative.