If the app does not show on your Watch after purchase please go to App Store on your watch (not phone) - scroll to bottom - Account - Purchased - and download from here.

WatchSketch allows you to draw on your watch without needing your phone in any way.
The whole screen is you canvas, you can draw on any part of it.
You can change color and size of the brush.
Saving the drawing as simple as it gets: by pressing the 2 side buttons of you watch it takes a regular screenshot and it is saved to the photo library on your phone.
You can also share the drawing by message or mail.