I'll try to make this as basic as I can but I get sidetracked.
Motorcycle on center stand.
In the old days we had a simple method of balancing the carbs on an Airhead. We pulled the plug wires off the plugs and rested them on top of the plugs so they still made contact but could be lifted to break contact. What we are going to do will have the same result but we are no longer allowed to have an open secondary in the ignition circuit. If the bike has electronic ignition this can and probably will cause several hundred dollars in damage but even if the bike has the old points and condenser we are told it may cause damage. Therefor we have made the shorting rods and with their use either side can be stopped from firing but instead of an open circuit the secondary fires into ground and no damage is done.
The ferrule from the original plug package is threaded onto the end of the screw and held in place by an extra small nut. Enough threads in the ferrule are left so this can be screwed onto the spark plug. It should be tightened a little because the bikes vibration can cause it to lose contact or fall off. The plug wire can be attached to the other end of the small threaded rod. The bike will run like this on the center stand rather well. During the process if you want to take a spin or something I recommend taking the rods off because they can become loosened on the road and are hard to deal with while negotiating traffic (don't ask how I know this).
The bike was previously warmed up with a 10 to 20 minute ride. With the engine idling a screwdriver is used to short either cylinder's plug to ground and the running cylinder on the other side is noted. The bike may continue to idle on one cylinder or it may die. It should sound the same, same rpm, same number of thumps before dying. To balance the carbs use the idle speed screw. You are looking for balance.
Total rpm or idle rpm is only a function of both carburetors working together. If you have the carbs balanced but idle to low then both need to be raised.
Idle mix is much harder to explain and I have trouble with this. I use the starting number of turns listed in the Bing book. If I am having a racing idle that is too fast I probably have them set too lean. Very small increments of movement seem to be a lot of change sometimes. I prefer to err on the side of too rich rather than too lean.
The shorting rods are used for setting idle balance. The carb cables need to be balanced also. This is balance for everything above idle. Idle is only when the throttle is completely at rest and there is no play of the throttle cables. Once the throttle is twisted and the cables engage idle is over and has no bearing on what the engine does. To balance the cables I watch both carbs at the same time to see their butterflys open at the same time. OK, I can't watch both carbs at once but I'm just trying to say that is the idea, that both butterflys open at the same time. There should be some slack in the throttle cable. If the part of the cable where it goes into the threaded screw on the carb top is pushed to the side or lifted out of the recess there should be the same amount of slack in the two cables. Sometimes a bit of unbalance in the cables is hard to pinpoint because they don't seem to be the same internally or one of them seems kinked in the sheath. It can take a lot of fussing to get all this right.
There is another recent thread where I tried to cover all this recently. It may be just another telling of this story and it may be a better or a worse version but I refer you to it just for redundancy so I hope it helps.
Takes you to the start of this thread. My telling of the carb balance method is post #240. Also check #243 and #248. There is also a lot of other info in Tete's thread about timing and other stuff.
I hope this helps to get you started. Next Spring I want you to tune my R90/6 for me.