Creating a modded server (Short version)

Forge Server

Make a new folder somewhere. It will be referred to as your server folder in this guide.
Download the Forge Installer from https://files.minecraftforge.net in your Web browser. Note that you will need to choose the same version as your client modpack uses.
Run the installer, and select "install server". Click on the ... icon and select your server folder. Note that running the installer requires having Java installed.

For anything below 1.17, java 8 will work.
For 1.17, you will need java 16
For 1.18, you will need java 17
For 1.19, you will need java 17.0.3 (In general, for versions past Minecraft version 1.13, newer javas will also work.)

Copy over the mods, config, and, if any, mod asset folders; like for example flans from your client modpack.
Remove all client-side-only mods, if any (and their corresponding config folders).
Check if there is a run.bat and run.sh file in your server folder. If yes, open them and read them, and skip to 8.
Create a start script for your server in the server folder. If you're on Windows, make a .bat file. For example: run.bat If you're on Linux or Mac, you'll want to make a .sh file instead.
The contents of the file should be something like: java -Xmx4G forge.jar nogui Where you replace 4 with however many gigabytes of ram you want to allocate to the server, and forge.jar with the full name of the installed forge. If you're on Windows, you will also want to add pause on a new line.
Run the start script (the .bat if you're on Windows, the .sh for Linux or Mac). This should generate an eula.txt file for you to open, and agree to, by replacing "false" with "true". Then run the start script again to start the server for real.
If you want others outside your WiFi to connect to the server, you'll want to portforward, or alternatively, use a Port Tunneling service instead, and then restart your server.