The big nut in the idler pully needs to be losened first with a 24mm ring spanner. The little nut that has a funny shape is actually a double hex and from memory fits a 13mm double hex socket. The little one is the one that will move the pully up and down.
If you dont have a 24mm ring spanner use a socket to losten the power steering but you will need a ring spanner and a 13mm socket at the same time when reinstalling it as you need to keep the tension on it as you tighten the big nut again. I am naughty and use a pair of vicegrips on the big nut
Incidentally the alternator and waterpump belt is the other way round - the little nut is to loosen and the big to adjust.
Hope this helps