Be as descriptive as you can...it's far better than just saying things like "nice head" or "good aroma" or "I like the flavor."
What is it about the head that looks nice? is it Frothy? Dense? Mousse-like? What about the aroma? Is the aroma hoppy? Malty? Caramel-like?
What flavors do you perceive...malt? hops? roast? wheat?
Having said that, I feel that the hardest part of judging is finding the right descriptive language to describe what you perceive, so don't feel awful if you have a case of writer's block.
Developing a good "judges vocabulary" will take time and experience...and will always be a work in progress.
Oh, and don't be too serious about it, it should be a fun thing to do...not something like a trip to the dentist! (no offense to any dentist's on the forum!)
More good stuff. I might just look like I know what I'm doing.
How many beers are typically judged in a session and how long do you expect we'll have per beer? They said they have more entries than last year, but they've enlisted more judges.
EDIT:
Never more than 12 beers to a flight...typical might be 10. A good pace is to do one entry in ten minutes. Or at least in my experience...