Say you want to download this face:
Then, on your watch, open the browser and navigate to the thread for the watchface. Then just follow the download link and choose download. Then open the file manager and navigate to the downloads folder, long press the zip file and choose either cut or copy and then navigate to the clockskin folder and paste it in there. If you’re using UL that should be all you have to do, but if you’re using the stock launcher you will have to unzip the face as well