There are many ways to set up terrain, and any method is perfectly acceptable as long as it creates a fair battlefield. The missions all advise you to set up the terrain in a mutually agreeable manner, and any of the following methods would suffice. Remember that the purpose of setting up terrain is to provide an entertaining and interesting battlefield, not to impede movement or lines of fire so much that it becomes almost impossible to actually fight the enemy. If a piece of scenery is going to be a major feature of the battle, such as a wide sulphurous river which cannot be crossed, then you need to OK this with your opponent. This sort of battlefield is perfectly fine, and might make for an enjoyable game, but you and your opponent would have to agree it beforehand. Here are some methods you might employ when setting up your battlefield:
Placing Terrain on the Battlefield
Source: Core Rulebook 3E, p. 124
There are several acceptable ways to set up terrain before a game, as long as the result is a fair and entertaining battlefield.