If you want an instant result ... poke every hole in the shower head with a paper clip and then scrub it with a nailbrush.
The descaler will do a better job if you have time on your hands.
You could also put viakal all on a sponge and squeeze it over the shower head and leave for a while it's quite thick. Then use a stiff brush on it to get rid of the scale.
Oh, what a palaver, I've yet to use my new shower installed a couple of months ago with the bathroom suite, and now I know I'll just stick to bathing as per usual.