I did the following:
- I went to the buddywatch site linked to above (although there is an app, you can also go to heir website)
- Found watch faces I liked, opened the page for the face, and downloaded the face using the Download button.
- Moved the watch faces I downloaded to a folder in my iCloud Documents folder (as long as it is a folder you can get to when synced to your iPhone you should be good).
- Went to that folder on my iPhone using the Files app.
- Clicked on the watch face I want to load to my watch.
- This takes me to an "Add Watch Face" screen. If the watch face isn't supported on your phone you will see a message to that effect (I got it for 2 Nike faces I downloaded), otherwise you will see a preview and an "Add to My Faces" button
- Click on the "Add to My Faces" button - this will add it to the My Faces section of the Watch app on your iPhone
- NOTE: if there are complications in the face that you don't already have, you will get prompted to purchase or download the complication.
- You can now, if all goes well, go to the My Faces section of the Watch app on your iPhone and add the new face, just like you would add an Apple face.
I tried all of the above steps for 2 or 3 faces except for the final step (loading it to my AW6 using the Watch app on the iPhone). I just got my first AW6 2 weeks ago and I really like the California watch face, so I don't have an urge (yet) to add more faces.
I hope this helps - there is a lot to learn about the Apple Watch, as I find out all the time.