Open your radial menu with the button and select "Potions" or "Poisons and Traps" and push . If you selected "Potions", select "Herbalism" and if you selected "Poisons and Traps", select "Poison-Making". If you have the necessary materials, you'll be able to craft an item here. You'll find the materials scattered throughout Fereldan. Search bodies and plants for the materials. You'll find some for sale from merchants as well.
Below are the potions, poisons, and traps you can make with the first level skill. As you move to Improved, Expert, and Master, more recipes become available. These are the easiest though. Be aware that you'll have to buy flasks from merchants.
Lesser Health Poultice: 1 Elfroot, 1 Flask
Lesser Lyrium Potion: 1 Lyrium Dust, 1 Flask
Mabari Crunch: 1 Elfroot, 1 Deep Mushroom
Venom: 1 Toxin Extract, 1 Flask
Deathroot Extract: 1 Deathroot, 1 Flask
Rope Trap: 1 Trap Trigger
Small Caltrop Trap: 1 Metal Shard
Small Claw Trap: 1 Metal Shard, 1 Trap Trigger
Small Shrapnel Trap: 1 Metal Shard, 1 Trap Trigger