As the title, I can't seem to work it out. I reckon I'm just too old!

If you have an infirmary, open that up and you can select players you want to heal from there. You can select multiple players on this screen.

If you do not have an infirmary you can open your strategy or training building, select your player that you would like to heal, select the Health tab and apply healkits from there.

If these are blanked out it means the player health is at maximum. Ensure that you do have heal kits by joining a guild and ensuring you ask and send healkits