Replacing an electric shower with exactly the same model myself

[FondantFancyAnyone]

This may be one for @PigletJohn, please.

Can I replace an electric shower with the same make, wattage and model myself?
I think that as long as I am careful and follow the instructions to the letter, it should be very easy.

The shower is on its own electricity circuit, and I the stopcock is closed.
Do I need to let the hot water run until the taps are dry before doing it?

I think the valve in the old one has broken. The shower still works but the water still runs after switching it off (or it would if I hadn't stopped the water and lecky)

I have no plumbing experience, but I have replaced a light fitting, can change a plug etc.

What could go wrong?

(Any advice welcome, but I'm an anxious type, so no scare-mongering please.)

PTFE tape will take up a bit of slack, this might be why some older plumbers sneer at it because they claim* all their joints are a perfect fit first time.

*this is not true.

Thanks, @PigletJohn . Not sure what you mean by "When you are using two spanners to turn a nut on the fitting without stressing or twisting the pipe, I find a good way is to position the handles of the wrenches so you can span them with one hand and squeeze them together."
Do you have a video link, please?
My search engine is being unhelpful. The NHS does not replace brass plumbing elbows!

So I need a bag of olives, PTFE tape, and two elbows? Do I need anything else?

The old elbow is unlikely to be damaged, so if you clean it, reassemble with tape, and tighten it, it will probably not leak.

Old jointing paste needs to be cleaned away.

Cross-posted.
Both elbows are leaking. The image is from a video. The blue mark are my unartistic depiction of water leaking from both elbows.

Let's suppose you put the two wrenches on the nut and fitting, and you position the two handles so that, on a clock face, they would be at 11 o'clock and 12 o'clock.

Let's suppose that your hands are big enough to span both handles and squeeze them together, so that the nut tightens.

The force on the two handles will be equal and opposite, so the pipe or shower will not be twisted.

Here's a bag of olives.
www.screwfix.com/p/flomasta-brass-compression-olives-15mm-20-pack/69557

The blue mark looks to me like it is leaking past an olive that is not tight, or is missing, or in the wrong place.

An old buddy who was once a Marconi engineering apprentice told me that in the workshop, they were told to drill and tighten bolts on scrap engines until they got the feel for how tight is tight. This included having some that were too loose and fell out, and some that were too tight and broke. I don't how how to teach the feel. In important work you use a torque wrench. Plumbers mostly do it by feel.

If you have a spare elbow on the worktop you can try tightening it until it won't go any more. Ordinary spanners are sized so if you use the strength in your wrist, it will be about right. So big spanners are longer for more leverage. Small spanners are finger-sized.

Sorry, I don't understand that at all. @PigletJohn

Do I not need to remove both elbows, clean them with a washing up plastic scrubber, then put the elbows back on, put the nuts and olives on the pipe before I tighten the nuts?

How do I loosen the nuts to get the elbows off?

Yes, if they are old ones.

I call it a green nylon pan scourer. It will polish up copper like new.

Piglet John you have the patience of a saint!

You unscrew the nuts away from the elbows. The nuts will not come off the pipes because they are trapped by the olives. But they will slide out of the way.

Like everything else, it's easy once you've done a few.

If you still have the old shower you could practice on that.

I'm completely lost.

Could you point me to a video please?

What should I type into the search engine to get a clear video showing a brass elbow being removed from a copper pipe that is already plumbed in, please?

This one shows the using the PTFE tape.

What I'm missing is a video showing using two wrenches when undoing the nut so that the pipes don't move about when I do it.