How do you set idle screw?

Crank the engine, let it idle. If it dies, turn the screw in 1/4 turn. Crank it. Repeat until it doesn't die. Then turn it in one additional 1/4 turn.

Now if you mean the mixture screw, typically 1 1/2 turns out. After you set that, rev it up. If it revs clean, leave it. Another method is to set the idle screw and set the mixture screw 1 1/4 turns out and back it out in 1/4 turn increments until further adjustments cease to increase revs at idle, then turn it 1/4 turn in.