To find your instance please follow this guide: https://www.creeperhost.net/wiki/books/minecraft-java-edition/page/finding-your-instances
On the instance page of the server you want to add a mod to click on
Mods in the instance navigation list.
Once on the
Mods page there are multiple methods to install a mod. Each method has been described below.
Click on the large textbox that says
Find On Curse and search for the mod you want to install
After you find the mod you want to install make sure to select a version that will work with the version of Minecraft running on your server and click
If you have the mod(s) that you want to add to your server already downloaded then you can click on the darker box that says
Click or drag files here to upload to your server
In the window that opens you can then navigate to where the mods you want to upload are located then click
Once you have the mods you wanted installed you will need to restart the server to enable them.