Although every player who has participated in even one tournament receives a rating from NASPA, a player must have played 28 games in the past 24 months to be considered "active" and have a ranking.
NASPA publishes a monthly list showing the ratings and (potentially) rankings of every player.
A player's 12-month ratings change is simply his or her current rating minus his or her baseline rating.
Find a player's baseline rating as follows: Of the month-end ratings lists less than 13 months old, find the oldest one on which the given player has a ranking (i.e. was "active" by NASPA standards). The player's rating on that month-end ratings list is the baseline rating.
Example 1: It is August 21st, 2011. A player was active on August 1, 2010 and had a rating of 1250 on NASPA's month-end rating list on that date. Her baseline rating is 1250.
Example 2: It is August 21st, 2011. A new player became active on January 8, 2011 (by playing his 28th rated game) and had a rating of 952 at the end of that tournament. He played two weeks later, on January 22, 2011, and reached 968. He had a rating of 968 on NASPA's month-end rating list of February 1, 2011. His baseline rating is 968.