Basically, all of the Pokemon you wish to enter (usually 12 and up) are arranged in you boxes from left to right, up and down, and a number from 1 to x (x being the number of competitive Pokemon) is assigned to each.  Then, your opponent selects 6 numbers from 1 to x.  The Pokemon you use correspond with the numbers he/she picks.  Then you do the same for your opponent.  If you ever want to Roulette battle, PM me.