Hello and welcome to PBreezey's guide for The Division achievement "One Down, Two to Go!"
To complete this achievement you need to have completely upgraded one wing of your Base of Operations. It is completely up to you which order you upgrade your wings in. I first upgraded Security, then Medical, then Tech. Working on this achievement will also give you progress towards:
Upgrades: Each wing of your base has a total of 10 upgrades available for purchase, and the total combined cost of the upgrades is 3000 supplies (each wing uses it's own supplies so upgrading the Security Wing requires 3000 Security supplies). To upgrade each wing, simply enter the wing in your base of operations and hold down
at the laptop located in each wing. From there you can see each upgrade, what benefits you gain from the upgrade, and which ones you still need to unlock.
Note: Some of the upgrades have a prerequisite of already owning an earlier upgrade. Make sure you have met all the prerequisites before attempting to purchase each upgrade.
Supplies: Supplies are earned by completing main story missions and encounters. Each story mission rewards you 500 of the supply type the mission is associated with; while each encounter rewards you 60 of the supply type the encounter is associated with.
Note: You DO NOT need to complete every single encounter to have enough supplies to fully upgrade a wing, though I do believe you will need to have completed each story mission. As you can see in the screenshot below (which I took as soon as I won this achievement) that there are still plenty of Security and Medical encounters for me to complete (I accidentally did more Tech encounters than I needed to, otherwise there would be 3 more of those on the map as well):
If you have any questions or comments please let me know. If you're going to downvote, please comment first and give me a chance to improve whatever issue you may have.
Good luck and happy hunting!