Two players sit with their arms crossed at the wrists, in thumb-wrestling form.
Each player counts off the standard, "one, two, three, four, I declare thumb war!" with both thumbs and the standard rocking of the thumbs from side to side.
Scoring works as follows:
One set consists of three games
One point for each thumb "captured" by yours, per game. Max score 6 per set.
Tiebreakers handled similarly to tennis - you must win by two points.