SMAPI (Stardew Modding Application Programming Interface) is a tool to help modders make changes to Stardew. It is a standalone executable which goes alongside your Stardew.exe.
- Firstly, make sure you have .NET 4.5. You can get it here: https://www.microsoft.com/en-gb/download/details.aspx?id=30653
- Download the the latest release binary here: https://github.com/ClxS/SMAPI/releases/latest
- Extract the zip file alongside your Stardew.exe, for example, if using Steam this would be somewhere like C:/ProgramFiles/Steam/steamapps/common/StardewValley
- To launch SMAPI, launch StardewValleyModdingAPI.exe
Mod developers would work off the release branch. The master branch will contain mid-version updates which could make your mods incompatable with both the current release and the upcoming releases.