Surviving the Aftermath
68 Achievements
GOG
Local Leader
Reach a population of 50
35.2%
How to unlock the Local Leader achievement in Surviving the Aftermath - Definitive Guide
Pretty self explanatory. I suggest doing this in a custom game with high starting population and catastrophes turned off. As long as you keep a well stocked medical facility, use good housing, provide adequate entertainment, and accept all groups of survivors at the gate, you will have a steadily increasing population.