The forum league signup form is now available: http://www.bowlingfans.com/league/2009/12/23/signup/

For book average, if you are going to be submitting scores from a league you bowl in, enter your current average in that league. If you'll be open-bowling to submit your scores, use the average of your last 9 games that you bowled for score, and try to bowl on a consistent day/time at the same center.

If your league uploads scores to League Secretary or a similar site, you can include the link in the Notes field.