I wrote up how I always do it and it was included in the FAQ's while those were still being updated. I use fingernail polish to mark the pulleys and the belt. You have to be careful to duplicate the marks exactly when you transfer them to the new belt.

Indexing the Timing Belt Markings. [Tip from Randy] Lets assume the belt that is currently on there is correct. All you want to do is replace it with a new belt- you want all the orientations to remain the same. Mark the current belt in some permanent way in relation to the gear. If the timing marks on the gears match up with an individual cog on the belt that will be perfect- if they don't you might have to make your own marks on the gear (fingernail polish comes to mind). The point is once you have the belt indexed to the gear arrangement you can easily remove it and you could put it right back on and be confident of getting it correct because you have the index marks on both the belt and the gears. So all you have to do is take the indexed belt off and carefully transfer your index marks on the old belt to the new one. Do this very carefully and be sure you match cog for cog as you move around the belt and mark the new belt exactly like the old one. BE CAREFUL to double-check the markings before going back to put it on. You might want to mark the front of the belt for reference in relation to the front of new belt. Some belts are already marked when new, but I always mark my own.
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Any twenty minute job is just a broken bolt away from a three day ordeal