Like most stuff it's really simple. Wrap the cotton sewing tape around the bar. It stays put by itself long enough to get to shellacking. Use a 1/2" chip brush, preferably from dollar store, otherwise from hardware store. Use lots of shellac.
It's simple but there is always more to find out. My latest discovery is fresh mixed shellac from flakes is even easier to apply. And comes in colors. Just did some bars in ruby shellac, which is more of a chestnut or cordovan with highlights in red, blue, purple. Also you can get clear that is really clear for the white tape, pre-mixed clear is always yellow. Take a look at the galleries at cycles-alex-singer.com. There is a lot of calculation in the colors he is getting. Would have to tape a whole lot of bars to figure all that out. Oh, fresh mixed in exotic colors and suddenly not so cheap.