It's an app that Samsung's developing that's still in beta. You can do a Google search and find a link to download it, just don't expect a finished product.
May I suggest this guide?
ht tp://forum.xda-developers.com/gear-s2/general/gear-watch-designer-installation-guide-t3285291
It is very excellent. Reading thru the thread is also helpful even though it is lengthy.
here are two watch faces i made withe the gear watch designer app.
The gears are moving and the balance wheel swings back and forth just on a real watch.
Implemented some background changing and touch areas to open apps (see picture)
Look in the store for: "Classic Blue Rev2" and Classic Blue Star"