Practice it in pugs, a lot. Focus on having a good tone, being concise, clear, confident, and loud enough/understandable. Calling actions (like back up) is usually better than describing the situation (like we're down numbers), it's one less step the rest of your team has to think about when performing.
Also everybody on the team should generally call what they're doing, whether they're pressuring one area, backing up, supporting another player, etc.