To install Minecraft Forge on the Windows 8 operating system, first download the software: either Minecraft or Minecraft server and Minecraft Forge Universal; then run the installer. As of 2015, a complete setup guide is published at MinecraftForge.net.
After installing the software, use the launcher to open the software one time. It is not necessary to start a world or connect to a server; just bringing up the menu is sufficient.
Next, download Minecraft Forge Universal from the Files section of MinecraftForge.net. Start the download by clicking on the installer button in the Download Recommended table. Once the file is downloaded, open the installer by right-clicking the JAR file. Click on Open With and select Java.
To install Minecraft Forge as a client, first click the Install Client button, and then click the Next button. A prompt stating "Successfully installed Forge build" appears. The software then creates a profile titled "Forge" in the launcher. Click the profile to log in and begin playing. Minecraft Forge launches after the green bar at the bottom of the window fills completely.
For server installation, click the Install Server button instead of the Install Client button, and the installer will install the file minecraft_server.jar to the path specified at the time of installation. Double click the Forge server jar for the graphical user interface, or use command line for batch or shell scripts.