This all depends on how dirty the undercarriage is.
If it's like most project cars I've worked on with years of caked grease and dirt typically start with a scraper.
If we're past that stage I hose everything with a strong mix of Purple Power or Dawn dish soap and I let that soak then blast it with a pressure washer. After the pressure washer I'll do the brush boogie as mentioned by Turbo6inKY followed by a good rinse and dry with the air hose. Follow this with whatever temperature and surface prep your coating requires.
Obviously a rotisserie helps immensely with this method but I've done the whole thing on my back using wheel stands while lying on a creeper wearing a raincoat, safety glasses and a face shield.
