Files were tested on SMAPI version 4.0.4 and might not work with older versions, especially the Less Diverse Valley mod.

You need SMAPI. Download and follow the instructions
https://www.nexusmods.com/stardewvalley/mods/2400

After that, download Content Patcher, and follow the instructions
https://www.nexusmods.com/stardewvalley/mods/1915

Put all unzipped mods into the Mods folder. That's it, for these mods at least.
If you're using Steam, the Mods folder will most likely be under Steam > steamapps > common > Stardew Valley > Mods

Also, there's a specific readme for the Less Diverse Valley mod in that folder.