Downloading Mods
Make sure you know how to Create an Instance before attempting to download mods.
Inside the Edit Instance menu, select the version tab on the left sidebar. From here, you will need to decide on which mod loader you would like to use. Fabric and Forge are the most popular, with Forge having the most mods available. Fabric and Forge mods are generally not cross-compatible, so choosing between them will change your selection of mods.
Once you have picked out your ideal mod loader, go to the Mods tab, and select the Download Mods option that's to the right. From here, you can choose to install from both Modrinth and CurseForge services. Modrinth focuses more towards free and open-source mods, however, CurseForge has a much larger selection.
It is worth noting that many mods will be available on both platforms.
After you've chosen your mod provider, you can search or browse for any desired mods. Click the Select mod for download button to add each mod to your download queue. Once finished, you can now press OK, and your mods should begin downloading.
Fabric
If you have elected to use the Fabric mod loader, then please ensure that the latest version of the Fabric API mod available for your game version is installed. It is mostly required for Fabric mods.
If it is missing, you can find it by right-clicking on your instance in Prism Launcher and selecting the Edit Instance option in the menu.
Quilt
For installing Quilt in Prism Launcher, follow the quilt wiki.