Some scenarios include guidance on the terrain features to include and where on the battlefield they should be placed. Most of the time, especially if playing a simple pitched battle, the placement of the terrain is left to the players. The recommended way to do this is as follows:
Determine how many terrain features to set up, based on the size of your battlefield.
From the terrain available, each player chooses a number of features, up to the maximum number of features needed.
Once both players have chosen, the winner of a roll-off places the first terrain feature.
Players then take turns to place a single terrain feature from all of those chosen until the agreed number of terrain features have been placed. Any unused terrain features are set aside.
Players can place terrain features anywhere on the battlefield, but not within 12" of a terrain feature placed by their opponent, and not within 12" of the centre of the battlefield. The only exception to this are special features, which must be placed within 12" of the centre of the battlefield.
Should the players agree, or should the scenario require it, battlefield decorations can be placed in the same manner.