For Minecraft 1.6.x
Download and run the new Minecraft launcher at least once.
Download the latest version of Minecraft Forge API installer, choose the installer option.
Run the downloaded jar file. It will open a menu as shown below:
Press OK, and this will appear:
Open the Minecraft Launcher and select the new Forge profile
Enter your username and password and launch the game at least once.
Minecraft Forge will now have generated its folders in %APPDATA%/.minecraft. Place mods into your %APPDATA%/.minecraft/mods folder as you have in the past.
Run from the Minecraft launcher through the Forge profile and profit?!?!
For Minecraft 1.5 and below
- Open your Minecraft launcher.
- Open the “Options” menu near the “login” button
- Use the link in the Options menu to open the .minecraft directory
- Navigate to the bin folder.
- Download the latest version of Minecraft Forge, choose the universal option.
- Open minecraft.jar and the Forge version you downloaded with appropriate archive software. (i.e WinRAR, 7-Zip)
- Drag all contents from Forge into the minecraft.jar
- Delete the META-INF folder from the minecraft.jar
- Close both minecraft.jar and Forge download.
- Go back to the .minecraft directory.
For Windows users, you just have to run install.cmd. If you’re on linux, you need to copy files and apply patch manually for now, A install.sh for linux users too is on the way.
Credits: The Minecraft Team