Set the mix screw when it is fully hot. Start around 1.5 turns out. Have it running, but keep a hand on the throttle so you can blip it if it tries to stall. Turn the screw in sloooowly, counting turns(or fractions of). It will either want to idle better, or get worse. If it gets worse, turn it back to 1.5 and go out slowly, listening for a better idle. The idea is to find at what point the idle gets worse in each direction(lean and rich) and set it for the midpoint. Example; if you start at 1.5 turns out, and it won't hold an idle at 2.25 turns out, and won't at 1.25 turns out, then set it at 1.75.
Rule for idle mix screws; if the screw is downstream of the slide, more turns out = richer. if the mix screw is upstream of the slide, more turns out =leaner