1. You should have Stable Diffusion installed locally.
2. If it is, put downloaded model file to Stable Diffusion folder -> models -> Stable Diffusion
3. Start your WebUI instance and check the upper left corner (click on the list)
4. If you WebUI was running already, just click on "refresh" button near this list
5. Now you're ready to go with any model you've downloaded
After this just to be sure, check the Clip Skip value in Stable Diffusion settings tab -> Stable Diffusion on the left list -> scroll down and use value of 1 (if you're using a realistic models) or 2 (if you're using an anime model), and be sure to check authors recommendations about clip skip. That's it.