How can I get the battery icon on the face of my watch?

Read this to help you further

https://support.apple.com/en-gb/guide/watch/apd6ce85daf4/watchos

Press down on your watch face till it zooms out and says customize at the bottom of the screen. Tap customize. Then you’ll be able to edit your watch face design and characteristics surrounding it. Slide over to the right, to the last little white bubble, you’ll now be able to edit the other little attributes. Pick the area where you want the battery percentage to be shown, then use the Digital Crown to scroll through to “battery”.