Dual carbs:
You'll have the best luck doing it with the car at operating temperature. Too cold, and your settings will be off.
Step 1: check your timing. Timing is set at total mechanical advance. If you have a vacuum can, remove the hose and plug it. Set the timing to 30* (+/- 2*) total. That might be 2500 RPM, or it might be 3500 RPM - it's all dependent on the kind of day the political prisoner in the Chinese reeducation camp was having on the day he made it. Likewise where the advance lands at idle - it might be 5* BTC, or it might be 10*. You'll definitely like it better if you got a "Wednesday distributor" and it lands at 10* rather than 5*, but you get what you get and don't throw a fit.
Once that's done, you can drop a down link and sync the carbs at idle. The guys that go to 800 RPM have a distributor that idles closer to 10*, the 5* guys are gonna want something around 1000 RPM, otherwise there's a flat spot off idle that no amount of futzing with the carbs will ever cure. Check all 4 holes. If trumpets on the same carb are sucking differently, open the air bleed a quarter turn or so on the low cylinder. Make sure the bypass screw is full closed on the high one. Maybe you'll get lucky and it'll work - it doesn't always (worn shaft bushings or twisted carb shafts are a thing).
Once they're really close, you can just use the number 2 and number 4 trumpets to sync. Get them as close as you can side to side. Once you're there, reconnect the linkage and check again to see that you haven't moved. You haven't really "synced" the linkage yet, but you should be idling like glass.
If nothing moved, you can go to the "just off idle" sync. Danny wants you to use the throttle pull because the hex bar deflects (even if it's ever so slightly) differently pulling from a different spot. I like Gordon's little tool idea a lot, but I'd set it up so it's as close to the actual linkage pull arm as humanly possible. Going off of one drop arm or the other is going to leave you unhappy.
I don't go by what RPM I sync at so much as by where I am in relation to the throttle stops. If I can pull the throttles just off the stops and sync the linkage there - it's perfect.
Here's an insider tip - the drop legs are where everybody gets lost. The heims can be really sticky after a while, and the treads on the drop legs themselves can get nicked up and might not turn freely. The only way this works is if the RH and LH threads move into and out of the heim joints freely and at the same rate, and if the heims themselves have a full range of motion. If one side of the rod wants to thread easily, and the other doesn't, you'll struggle. I run a 10-24 die down the RH threads all the time, but I don't have a 10-24 LH die. I should get one. It's important.
I've gone so far as to fill a hole on a linkage arm with epoxy, then redrill it about 1/16" closer to (or further away from) the hex bar because the total pull wasn't the same side to side. But as everybody keeps saying - once you're past about 1/4 throttle (probably much less), it really doesn't matter that much. It bothers me though. It might bother you too.
Sync-link solves all of the geometry, flexing, slop, and down-link nonsense (of course), but almost nobody uses it. You probably don't either. Sync-link is a nice bit of kit, but it ain't cheap. The hex-bar linkage can be made to work exceptionally well. It's just more work.