First, you need to know the survivor's skill Deadly Gift. When you select that as your secondary skill, a menu will pop up with icons of each type of trap in your inventory, for you to select one. After that, you just right click on the ground somewhere to set a trap.
Scorpions will wander around; if you want them to follow you, you need to use the Aura of Command spell on them. Some people have reported that using Aura of Command cuts the experience points from scorpion kills (or summoned / resurrected creatures) in half, while others found there was no difference.
