To begin, randomly choose a player. That player decides whether to go first or second.
The player going first chooses a side of the table they will deploy their army on. The other player gets the other side of the table. Then, starting with the first player, each player takes turns placing a unit in their deployment zone. The deployment zone is the area within two sortie base lengths of a player's side of the table.
The rulebook has specific restrictions on unit placement.
The typical table is 4 feet by 6 feet, with the deployment zones along the short edges. Since some units have a striking range of 5 feet, it would be extremely dangerous to put the deployment zones along the long edges.
Once all units are placed, the first player takes their turn. The first turn of the game is called the ambush turn. In this turn the player going first gets half the normal order points (4 order points in the standard game). In each subsequent turn of the game players get the standard number of order points (including on the second player's first turn).
The shortened ambush turn helps to mitigate the advantage of going first.