Each player who has no stack locked in combat rolls 2D6 (resolve ties). The player with the highest roll plays the player with the second highest roll and so on. Should there be an odd number of players the player rolling lowest will not fight and can put the spare time to good use by making coffee, taking a nap or a making nuisance of himself.
Each player paired off will determine the stack he will use in the battle randomly (I'm sure you find a way). For recruiting and withdrawing the stack will use his own tile as the battle would have taken place there.
To determine the terrain for the battle both roll a die and the battle will be fought in the tile of the player rolling higher. This player is also considered the defender.
To keep players busy, it will be made sure that every player will fight a battle even if none of his stacks is locked in combat. The random draw of stacks provides "weaker" players with the chance not to have to fight the "strongest" stacks of the "strongest" players every time.