I don't have a factory manual for your exact model. The procedure varies a little compared to a 1000 for example. I'm sure if you search or make a specific topic, someone can help.
Basically, there's a "nick" in the slide that you can use to get the slides APPROXIMATELY in synch. Turn the carbs so the nick faces you, loosen the adjuster holder nuts above of the slide assembly, then slowly turn the adjuster screws to get the slides uniform. Once uniform, tighten the holder nuts. A bench synch in my opinion is only to allow the bike to start up in preparation for a proper vacuum tool synch. Yes, a careful bench synch is better than nothing, but as I found out decades ago, you can bench synch all you want but it's no substitute for a vacuum synch. There's almost always at least one cylinder that will be off compared to the others. The vacuum level even effects the mixture entering that cylinder. One cylinder will run richer or hotter than the others, etc. Once you do a good vacuum synch you'll be amazed at how much smoother the bike idles, cruises, etc.
As for bench synch specifics, there are differences so I can't tell you 100% what you need to know. For example, I believe the factory manual mentions the '77 carbs have a "master" carb that needs to be synched first and you match the others to it. Also, your specific carbs may need to be synched not only for the closed position, but also for the fully opened position. Yes, a slide can be adjusted too far "up" also.
I use a Carbtune vacuum gauge. It works very well and there's no more mercury to deal with. It uses metal rods. It's been awhile since I bought mine but mine was about $110 about 3 years ago. The VM carbs will gradually lose their synchronization especially when ridden hard. I'm guessing the factory manual says to inspect or synch them every 5,000 miles or so? If you keep the bike or expect to have other bikes with multiple cylinders, the vacuum tool will be a solid investment over time. It can be used for 2,3, or 4 cylinders, even fuel injected bikes.
Just be sure after bench synching to adjust the idle control knob that lifts all 4 slides a few turns after you see the tip make contact with the throttle pulley. If the slides aren't up a reasonable amount at start up, the bike won't start or will stall. Warm the bike up fully, use 1 or 2 fans to avoid over heating, then adjust the side air screws using the highest rpm method with a base idle of 1,000 rpm's. Set the pilot fuel screws as I said earlier as a starting point. Vacuum synch on the center stand. Enjoy.
