That not an easy thing to do. Assuming you've read the Old School Primer here are a few extra things you'll need to know.
#1 Select a system that facilitates the rulings not rules play style.
Players in games using more modern rules rigorous systems often expect the rules to be used and resent a more freewheeling style. You can solve this by selecting the right system. Any of the retro-clone will be fine as will any other game your group prefers that meets this criteria.
#2 Think on your feet.
if you can't roll and go, you'll have a hard time keeping up with your players. As an old school GM, you'll find you'll need to be pretty creative, maybe more than you would in games with more detail.
#3 Keep a DM's Notebook
This can be a a traditional spiral bound notebook or an .odt document or whatever but keep notes. This will help you keep you game consistent and fair and save your imagination some where and tear when your reference stuff that went before.
#4 Strive to be fair consistent and trustworthy.
and last, most important. Be Excellent to Your Players (and San Dimas High School Football Rules ;) )
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