Unfortunately there is a lot more to perform. I just did it myself, because Huawei's developer support for testing watch faces directly on the watch is pretty poor.
This is no tutorial, but just for you to get the idea:
* Buy a cheap, used smartphone (Nougat+), because chances are, that you end up in a boot loop plus you might not want to hack your daily phone.
* Do whatever is necessary to install a custom recovery like TWRP (bootloader unlock, root, ...) (although there may be alternatives to install xposed from within Android). Once you have TWRP you may be safe, in case the next step bricks your phone, which happened to my Zenfone Zoom. I was still able to boot into Recovery and revert my changes.
* Install the XPosed framework. (can cause bootloop)
* Install a WatchFaceEnabler module for xposed (.apk)
* Install a beta version of the Health App (like 10.1.1.001)
THEN, this beta version will show a new menu item "upload watch face from phone" or so. Pair your watch with this phone/app and you can use this menu item to select the .hwt you exported and it will upload to your watch.