SMAPI is the mod loader for Stardew Valley. It works fine with GOG and Steam achievements, it's compatible with Linux/macOS/Windows, you can uninstall it anytime, and there's a friendly community if you need help. SMAPI is required for most types of Stardew Valley mod.
Install SMAPI See the player's guide to modding (or skip to install steps). To update SMAPI, download the new version and run its installer; it'll clean up the old version automatically.