Tap a unit's portrait to select it, then tap an open area on the field within the perimeter to drop a single soldier or vehicle. You can also tap then hold to place a group of them. Each type of unit has a preferred target that they will focus on over other enemies. Do not place your units too close together to avoid explosive damage that will hit all of them. Queue-up more soldiers and vehicles before a fight when you are full to prep yourself to begin training as soon as it is over.