This guide, while still providing decent information and resources, is outdated. We recommend Phoenix's Beginner's Guide as an updated alternative.
You may have made some basic LODs already. But you want terrain, so your landscape textures match? You want a cool map? You want grass? You'll need this.
The Animonculory has published an advanced LOD Guide which covers absolutely everything. There is also a separate grass LOD guide (hard on FPS),