In order to unlock this achievement you need to improve the planetary conditions using Terraforming machines and Special Projects that improve 3 Basic planetary conditions; Atmosphere, Temperature and Water.
Water is easiest to improve by building lots of Huge Lakes, just make sure you have the Water Production facilities to run them. (Temp at 25% Minimum required for Liquid Water)
Temperature is improved by using GHG Factory and Core Heat Convectors.
Atmosphere is improved by burning Waste Rock in a Carbonate Processor, having a Magnetic Field Generator also slows atmosphere loss.
Once you are at 40% Atmosphere, 40% Temperature and 10% Water the Clear Rains will start to fall and the achievement will unlock.
More detailed Terraforming info can be found on the wiki located here.https://survivingmars.paradoxwikis.com/Terraforming