Step 1: Ensure your server is stopped.
Step 2: Upload the mods and Forge files (usually come with a modpack) via a FTP client such as Filezilla, please refer to our FTP tutorial here. Alternatively you can zip the files and use the 'Archives to upload and unpack' option on the web FTP upload tab.
Step 3: Go back to your server page on Multicraft and select the 'Custom Modpack/Jar' jar dropdown option and then rename the Forge jar in your FTP to custom.jar.
Step 4: Start the server and you're all set!