Base idle is the idle speed with the IAC port blocked. I'm not a big fan of adjusting base idle without a good reason, like engine mods. There may be a case to do so if the engine has some wear on it but i would look to other things first.
Before attempting any adjustment, ensure that everything else is right first, like engine tune items. The throttle body should be clean(ed) as the amount of air flowing past it, which is what is being changed by moving the throttle stop screw, is affected by build up on the edge of the throttle plate. Cleaning is best done with the t'body off the car and the Idle Air Control motor removed (the IAC is the cylindrical component attached by two Torx screws). Use a rag soaked in solvent, kerosene works well, to wipe the inside of the t'body, IAC air port, IAC motor tip and throttle plate. Take care, as it is possible to bend the throttle plate. Using carburettor cleaner from a pressure pack, as is commonly done, can result in the grease being washed from the throttle spindle and IAC motor spindle, so don't.
The green cover is an anti-tamper device for the throttle stop screw which is what needs to be moved to adjust base idle. if it is missing, it means that someone has already moved the screw, generally to a position other than where it should be.
If you still think that you need to adjust the base idle, remove the green cap and mark the positon of the screw so at least you can go back if necessary. You will need a tachometer. The ones in the dash are not very accurate, so an external one is best. Many automotive multimeters will have this function built in. With the throttle body (and IAC) back on the car, warm the car to operating temperature and bridge the diagnostic terminals on the diagnostic link under the steering column. This locks the ignition timing to 10 degrees and prevents the idle being changed by the ECM altering ignition timing. Remove the inlet air duct and elbow. With all accessories turned off, including the engine cooling fan (you will have to wait as it cycles on and off), block the IAC air port with a finger and adjust the idle screw, if required, to achieve 450-550 rpm. Note that the engine will almost stall at this speed and you will burn your finger on the throttle body
. Put everything back together. Expect the idle to flare when you first allow air past the IAC port, as the ECM will have been trying to correct for the low idle speed caused by blocking it. That's pretty much it. You can allow the ECM to relearn a base IAC motor position by idling in the various combinatiopn of A/C on/off, drive/neutral but just driving the car will achieve that.