You need a working game on steam. Then you install SMAPI and it will create a launcher on the desktop (with a fun icon) and use that to launch the game with mod support.
Mods are put inside "Mods" folder inside game files - in Steam on Stardew game page where you have the launch button - on the right side there will be options and one of it to explore local files - click that - it will open File Explore and you should see "Mods" folder within game files, put mods there. Nexus that hosts mods also has a manager app that can download and put mods there automatically.
