Talk to the ally carrying the resource to start the mission. A marker then appears at a random location on the map. Escort your ally there, clear the area of zombies and let them (your ally and a member of a neighboring Enclave) do the trading. Afterwards, escort the ally home to complete the mission.
If you exit the game and load it, the ally carrying the resource and the trade location are randomized again.
Again, this is a good opportunity for a free backup. Take this opportunity to train skills, explore area, clear Hordes and Infestation, or gain some trust with your ally if they aren't your friend already.
If the ally dies during this mission, you always lose both the resource intended to trade away and the one you get in return.