It's easy, and it doesn't take much. Depending on how hot/humid it is, you'll use between a 1% to 5% mixture.
Lacquer dries by evaporation, so the hotter or drier it is in your paint area, the faster it dries. Get a high-temp low humidity day, and the stuff will practically dry coming out of the gun. Since it skins over so fast, air and moisture gets trapped under the surface and that's what causes the dreaded "blush".
Retarder slows down (retards) the evaporation speed of the vehicle/solvent/thinner (acetone) by raising its boiling point, so the lacquer has a chance to get on the target and stay wet long enough for any microscopic air/moisture bubbles caused by atomization and/or condensation at the gun to escape.
A shot glass is close enough for rock 'n' roll when mixing, as it's not hyper-critical. 1 shot roughly equals 1% in a gallon. I usually mix by in a quart jar, so I'll do half lacquer, half acetone, and a shot of retarder. Gives me a roughly 50% lacquer retarded 4%, which works out pretty good with my rig and my painting style in moderate humidity and upper 80s to mid 90s temps. But, I'm sure everybody does it slightly differently because you have different spray guns, different pressures, different painting styles due to pattern selection, etc.