Here's a video on how to obtain ALL of the prologue achievements, and more! Links to the specific achievements, in order of when you'll obtain them, are in the video description. I didn't show battles as some are walked through by the AI, and most others are simply auto fought and won. The last one appears very difficult, but my game cut out, and I didn't complete the prologue.
As for this achievement:
1. Once you get to Loki Palace, whether you've built or not, fully building a settlement should be within reach.
2. You require population stability within each fort/outpost/settlement, which is gained by building secondary buildings that have a "population boost".
3. Build a population boosting building, and wait multiple turns for the population to go up.
4. When it hits 1, you can build the next settlement upgrade, on the main settlement building, resetting the population score to 0.
5. Wait again for it to hit 2, and then build the main settlement building up to the last tier.
6. Wait a turn for it to build, and the achievement is yours!