The absolute simplest is a large kettle over a fire. You rig an overhead bucket/cistern of some sort. Top it up with
hot water from the kettle and cold water to get the temperature right. A short length of pipe beneath with a valve to turn flow on and off, along with a
shower head. Job done.
You can buy shower bags that you can hand from a tree which have the shower head integrated into the bag.
Personally when we are camping out in our own woods we have a fire running pretty much all the time, and it usually has a big kettle on it for brews.
Outdoor showers are lovely, but you want a bit of screening for privacy and some kind of floor so you don't end up showering in mud. 4
pallets placed in a square works well as a floor (maybe hit them with a sander to get rid of the worst of the splinters!).
When I used outdoor showers in a group setting the shower space was 100m or so away from the camping area (privacy through distance) and was in a natural thicket of yew bushes for decent screening. Simple way to prevent embarrassing meetings at the shower is to only have one shower bag which you take from the central fire to the shower spot when you need it. Bring it back when you are done. If the bag is missing you know someone is showering.
NB: you might be able to use a
solar shower bag with hot water from a kettle, but they seem to all have narrow necks for filling. The one I have used was more like an open top bag so you could pour a
bucket or a kettle straight in.