Install SMAPI, the mod loader required for most Stardew Valley mods
Find a trusted mod site such as Nexus Mods or the official Stardew Valley mod forums
Search for the mod you want
Check that the mod is compatible with your game version
Download the mod file
If the file is compressed, extract it using a zip tool
Open the mod folder and read any included instructions or requirements
Copy the mod folder into the Stardew Valley Mods folder
Launch the game through SMAPI
Verify the mod is working in-game
Keep mods updated when the game or SMAPI updates