Install
Install the latest version of SMAPI and SpaceCore.
Unzip the mod folder into <font style="font-family:Courier New;">Stardew Valley/Mods</font>.
Run the game using SMAPI.
Use
Start a new save (or load an unmarried save), and when the day starts you'll see a menu asking who you're married to. You'll start with the first house upgrade (so they have their spouse room), and heart events will still trigger like usual.
Compatibility
Compatible with Stardew Valley 1.5.5+ on Linux/macOS/Windows, both single-player and multiplayer.
See also