The players choose, or roll for, a half of the table each. They then place all the terrain in that half of the table. This is done purely by choice. Once both players are happy with the position of the terrain they roll a dice – the player with the highest score chooses his deployment zone as indicated in the mission.
Setting Up Terrain – Method 2
Source: Core Rulebook 3E, p. 124
Players split the table in half, each places terrain in their own half, then dice off for choice of deployment zone.