In case it isn't apparent, you have to raise each of the four terraforming statistics to 100 percent.
Atmosphere is easy to raise with carbonate processors, but atmosphere is also lost over time. Use the Launch Magnetic Shield project and the Magnetic Field Generator building to slow the rate of loss.
The GHG Factory building raises Temperature slowly but steadily. The Launch Space Mirror project gives it a substantial boost at the cost of a rocket and some materials.
Water is easy to raise with lakes. Small Lakes aren't worth the trouble, but all the rest are worth doing - the more the better, and the bigger the better. Also use the Capture Ice Asteroids project for a 5% boost.
Forestation is the most tedious statistic. You start raising it with Forestation Plant buildings. Once your statistic hits 40%, these buildings don't work to raise it anymore. Beyond that, the only way to raise it is with the Seed Vegetation project. Each use of the project raises Forestation by 5%, so after 12 projects, you'll have 100%.
- The Import Greenhouse Gases project boosts atmosphere by 2 and temperature by 1 and can cause toxic rains.
- The Melt the Polar Caps project increases water by 10 and atmosphere by 5. Read the on-screen instructions carefully before doing it; there can be side effects.