You go to Guild Master, ask him/her to modify/create a tabard (or crest, i dont remember), and then a small menù whit the guild symbol editing option appears. You can choose between various preset symbols, and change borders, color, symbol color, and border color. Preset symbols range between beer bottles to roaring lions. That's in short, described by an ex-WoW player. (quitted taking drugs playing a month or so, so i dont think infos are too outdated)