I just skimmed through the revised AD&D 2nd edition DM's guide and it says as long as you have the spell, you can make the scroll. However, you need to get special quills (from cockatrices, griffon's, etc.), use specially made ink (with magical ingreidents, like crodicle tears, etc.) in order to write it and then roll a percantage to see if it was successful.
To me, for the amount of magical scrolls that seem to float around in an D&D world, that seems like these things are easier to make then written. But that's always been the case for D&D/AD&D - there maybe an abundance of magical items to be found in the world, but if you want to create one, good luck...take more time, money, and energy than you can imagine and even then, you'll probably be unsuccessful.