Pick a round and enter your recent scores for it (X's optional). Your handicap is the average of your last 8 scores, so enter as many as you can. With fewer than 8 it shows as a provisional estimate until you log more.
Lower is better. A perfect round scores 0, and shooting X's on a perfect round takes it below zero. Handicaps are tracked per round and per bowstyle, but they all speak the same scale: every score is converted to its 720-equivalent, so your rating means the same thing whether you shot a 300, an indoor 600, or a full 720.