Inside the created folder you should find a folder called SMAPI 3.18.6 installer. Open it and run “install on Windows.bat”
Follow the prompts from the SMAPI installer to get the mod API added to your game, then continue here.
Once SMAPI is installed, in the folder that you unzipped you will find a “Mods” folder. Open it.
In Steam, Right Click Stardew Valley and choose “Manage > Browse Local Files”
Return to the folder that contains the “Mods” folder and copy it, then paste the “Mods” folder into the folder that contains your Stardew Install (which was opened in the last step)
Your install should then look something like this: