First read and follow the instructions under the download. Second here are the steps it takes.
Installation
These instructions work for both server and client.
- Download and Install Minecraft Forge Universal.
- Download Galacticraft.
- Download Micdoodle Core (not required for Minecraft 1.5.2 or lower).
- Drag the files you downloaded into the "mods" folder ("coremods" folder if you're using Minecraft 1.5.2 or lower) located in the minecraft directory.
- Click "Load Basic Components" when you start Minecraft.
- Restart Minecraft and play!