I was wondering how to adjust the idle on a 90 ls engine

 

there's a bracket on the intake manifold that your throttle cable runs through with 2 bolts on either side of a clip just adjust it from there

 

ttt

 

Maybe you didn't understand "JDMREX91" "He was say'n to loosen the 2 10mm nuts on the throuttle cable, then slide the cable either left or right till the idle is just right ( around 700-800 )
This process might that a could of try's till you get good --- It is pretty easy though

After I posted I notice that they might be 12mm nuts <--- Edit ---

 

Also check the Throttle Body for dirt and check the Idle Control Valve on the back the Intake Man. ( the one with Two Coolant lines coming out of it ) Spray it out with a carb/throuttle cleaner I would recommend BG cause it has a little lube in the Cleaner that is good for the Valve.