Step 1: Select the game server you wish to install Metamod/Sourcemod on. Step 2: Open the Mod Manager. Step 3: Install Metamod. (There are different versions, stable, stable-dev, and unstable.) Step 4: Install Sourcemod. (There are different versions, stable, stable-dev, and unstable.) Step 5: Start your server.
Note: Metamod must be installed before you can install sourcemod, and the version for metamod must match with the sourcemod version.
If you need to update your Metamod/Sourcemod installation, just uninstall both of them from the Mod Manager, and reinstall them you will automatically get the latest version available.