With carbs like the VMs that have individual throttle cables, you actually have to do two syncs, one for idle speed and one at a higher speed with the throttle open some. The idle speed sync is pretty self explanatory. You want both carbs idling at the same speed. You must set this first as I mentioned earlier. Your idle speed is controlled by a tapered screw bearing into an angled ramp cut into the side of the slide. Turning the screw raises or lowers the slide and that increases or decreases the idle speed. Doing this also adds or removes slack in the throttle cable. That's why this setting or sync must be done first, before the "at speed" sync. Make sure you have adequate freeplay in the throttle cables or the slides may not drop all the way, a too tight cable holding them open.
For the "at speed" sync, roll the throttle open slightly so both slides have opened some. Sync by adjusting the cable freeplay on one carb or the other. I usually get the idle tune and sync squared away then set one carb's cable freeplay where I want it (about 1/8" to 3/16" freeplay) then lock it down. I then do all the sync adjusting on the other carb's cable, matching it to my properly set first cable. It's not always possible to get a perfect sync at all speeds with individually cabled carbs, especially if the cables are old. Cables stretch with age and can do so unevenly. In a case like that, try to get your best sync in the 3K to 4K RPM range, the area you'll be running and cruising in mostly.
The mix screws on the VMs are air regulating screws, not fuel regulating screws like on the stock CV carbs. This means they act opposite to the CV mix screws. Turning them out adds more air and makes your mix leaner. As you may be aware, too lean of an idle setting can raise RPMs and even cause the engine to race. Keep this in mind when adjusting the air screws. If the idle speed keeps rising and rising, and you're getting to like 2 to 3 turns out, you've probably entered this "too lean" territory. Instructions for setting the air screws on BMW Bing slide carbs tell you to turn the screw back in 1/8 to 1/4 turn, richening the mix back up some, once you enter this increased idle speed area.