My methods:
If the brakes are already fitted and the cables are all set up so all your after is adjusting the pads. Pull the brake lever then hold brake lever in place with a cable tie, pads are now against rim allowing you to adjust.
If fitting for first time and not cabled up then fit wheel without tyre. Use an old brake cable placing the barrel nipple in one side of canti and cable clamped in other. Pull together and clamp with pads contacting rim and adjust.
Both ideas do not require third hand tool which just does as above, clamps the pads againts the rim for adjustment.
Personally I just use modern canti's with the v brake type adjustable pads, not in keeping with the aesthetic but work just as well and work with the levers etc.