First, you have to put the snapdragon on the mushroom that's on springs and bonk the mushroom so the snapdragon will bounce up to the window and grab the first part of the spell. If you look at it on the 'papercutter', the half scroll is backwards.

Then you go in the house and take the other half of the scroll from Dalboz' bedroom windowsill. That half of the scroll is also backwards. Then you go into the mirror and put both halves of the scroll on the papercutter thingie with the magnifying glass. The spell will right itself and Dalboz will tell you that you've got the spell but you can't gnusto it until you run it through a spellchecker.