WARNING: Please note that running server software on your computer without a clear understanding of what you are doing may seriously harm your system and lead to vulnerabilities.

Set-up:

1. Have you tested your modpack in singleplayer yet? If so, move on to #2. If not, test it.
2. Get a copy of a Minecraft-server.jar for whatever minecraft version your mods require (https://minecraft.net/download or https://mcversions.net/)
3. Get a copy of the Recommended Forge Universal jar version for whatever minecraft version your mods require (http://files.minecraftforge.net/)
4. Create a new folder for your server
5. Put the minecraft-server-<version>.jar and the Forge-Universal jar in the same folder.
6. Copying your mods folder from your modpack into the same folder
7. Remove any "client only" mods like Damage Indicators, Waila, Custom Lan Ports, etc.
8. Make a folder called "config" in the same folder.
9. Copy the contents of your client "config" folder into the server "config" folder.
10. Remove any config files for mods you removed in step 6.
11. Start the Forge-Universal jar like you would start a minecraft server.

Credits to rcmaehl and plowmanplow from the Technic Forums for the basis of this guide. 

 Note: If you are planning on hosting your server with a hosting company, it is recommended to contact your hosting support service on how to upload your new modpack server and proper setup procedures.