Step 1: Install Forge on your server
Forge is the server software that is necessary to load mods on your server.
Forge on Aternos: https://aternos.org/software/forge/
An alternative to Forge is Magma which is based on Forge and PaperMC. It supports the installation of mods and plugins together on the same server: https://aternos.org/software/magma/
Step 2: Install mods on your server
Aternos has a list with lots of different mods from CurseForge, currently, only mods listed on CurseForge can be installed on Aternos.
All mods: https://aternos.org/addons/
An example mod, DecoCraft: https://aternos.org/addons/a/curseforge/decocraft
Step 3: Install dependencies on your server
Some mods need other mods to work, these are called dependencies. If it's possible the dependencies are listed on the Aternos mod list, but sometimes you have to take a look at the mod descriptions to find all dependencies.
The example mod, DecoCraft, requires the installation of PTRLib to work.
Step 4: Install Forge on your computer
Most mods have to be installed on both, the server and the computer of every player. To install Forge on your computer, download the installer here: https://files.minecraftforge.net/
(If you use Magma on the server you still use Forge on the client)
Step 5: Install mods on your computer
Download all the mods, that you've installed on your server and put them into your mods folder (inside the .minecraft folder).
To download the mods, you can directly head to the mod pages linked in the Aternos mod list, for example:
Step 6: Start Forge on your computer
Start your Minecraft launcher and select the Forge profile, that was automatically created by the Forge installer (Step 4).
Step 7: Start Forge on your server
Start your server here: https://aternos.org/server/
Step 8: Connect to your server
If you have problems connecting to your server, you can read our article about that here.
If you want a mod to be added to the mod list, you can request it on the forums (https://board.aternos.org/).
Before you request anything, you should read this article: https://board.aternos.org/index.php?thread/7366-how-to-write-a-good-plugin-modpack-mod-request/