Dexter wants a recipe that 
Cor Kalom keeps locked in a chest in the Swamp Camp. Getting to it means talking your way through the camp with 
Lester's help, waiting for the laboratory to empty out in the small hours, and picking the lock. Buy your 
lockpicks from 
Fisk before you set off, because there is nowhere to get them once you are there.
The only hard part is the lock, so the real prep is having picks and a way into the laboratory.
Before you leave, find Fisk and buy at least two lockpicks. You need them for Cor Kalom's chest, and there is no second chance for them in the Swamp Camp.
Make your way to the Swamp Camp. Past the guards at the main gate you find Lester among a few others. Talk to Lester.
Tell Lester you want to join the Brotherhood, then ask him to help you get around the camp. When he asks where you want to go, choose Cor Kalom's laboratory, and he leads you straight there.
Follow Lester to the laboratory, go up the stairs to the top, and the chest you want is at the back of the room.
You cannot open the chest with people around. Even sneaking in gets you caught and makes them hostile. Find a bed and sleep until after one in the morning, then come back to the empty laboratory and step up to the chest.
Work the lock in this order:
- Fourth plate, left six times.
- Second plate, right five times.
- First plate, left four times.
- Fourth plate again, right twice.
- Third plate, right twice.
That opens the chest.
Loot the chest and take the recipe, along with anything else worth having inside.
Leave the Swamp Camp, head back to the Old Camp, and give the recipe to Dexter to finish the quest.