SMAPI is the Stardew Valley mod loader. It works well with GOG and Steam accomplishments, Linux/Mac/Windows is compatible, you can uninstall it at any time, and if you need support, there is a friendly community. For most Stardew Valley mod types, SMAPI is required.

Install SMAPI
Refer to the player's modding guide (or skip steps for installation). To update SMAPI, download and run the new version of the installer, which will automatically clean up the old version.