To unlock the My Little Fortress achievement in Far Cry New Dawn, you will first need to have recruited all five of the Specialists. All five of the Specialists will be recruited during Act 2 of Far Cry New Dawn as part of the story, so don't worry about trying to find them earlier.
Once you have all five Specialists recruited, you will need Ethanol to upgrade Prosperity's infrastructure.
You can gain Ethanol in Far Cry New Dawn in the following ways:
OutpostsThe best method for obtaining Ethanol is to capture an enemy Outpost. Once captured, you will be rewarded with Ethanol (bonus Ethanol if done in stealth), and then you can reset it for an additional 50 Ethanol and repeat. When you reset an Outpost, however, it will be taken over by higher-level enemies, making it harder to capture the next time around.
Once you reset an Outpost for the first time, you will unlock the Reduce, Reuse, Recycle achievement:
Highwaymen Supply DropsHighwaymen Supply Drops will be randomly dropped from aeroplanes. They will be noticeable from the yellow smoke and the red marker. When you locate one, take out the enemies that are guarding it and hold the Square button to loot it.
When you loot your first Highwaymen Supply Drop, you will unlock the Yoink achievement:

The Yoink achievement in Far Cry New Dawn worth 10 pointsLoot a Highwaymen Supply Drop. Be careful, it won't be unguarded! Ethanol TrucksOnce you have captured an Outpost, stand on a nearby road and wait for one of the random Ethanol tankers to pass by. Shoot the driver, kill any remaining enemies in the area, steal the truck and deliver it to the nearby Outpost.
When you successfully deliver your first Ethanol truck to a captured Outpost, you will unlock the White Gold achievement:

The White Gold achievement in Far Cry New Dawn worth 20 pointsCommandeer an Ethanol tanker and bring it back to one of your outposts.You will need to upgrade each of the following to level three to unlock the My Little Fortress achievement:
- Garage
- Training Camp
- Workbench
- Infirmary
- Expeditions
- Explosive Lab
- Healing Garden
- Cartography