How do you install a shower unit including plumbing/electrics?

unless you are a 13th edition qualified electrician I think that you are not allowed by law to wire an electric shower in. Get a professional in...safety first.
For these special multifunction units please follow the instruction manual, for simpler jobs see the how to videos in the store, on line, or borrow some from the library near you. Note: you need permits and inspection for plumbing and electrical work. Make sure you have someone with valid license to do the job for you or to check your completed work. Good luck.
Hey Harry you are a bit behind the times!! The latest electrcal regs are the 16th edition and you will also need part p building regs. Don't do it yourself you are risking lives, get a qualified electrician in.
It all depends on your house and the wattage of your shower unit. If it it is around a 10kw shower you are better to install a 10 mm cable from your mains that is covered by an rcd. You must fit a switch outside the bathroom or on the roof with a pull cord. The plumbing is one pipe from your cold water mains tap. You will not need a booster pump if your cold water tap has a reasonable pressure. A good tip is sometimes check under your sink usually that your main stopcock is fully open.