It really depends upon your DIY skills. Probably the walls are already covered with a waterproof material, but you will need to seal every corner with silicone mastic. Then you'll need to find a shower tray which is a close fit to at least three of the walls, so that it can be sealed at the edges. Then you'll need to buy a shower head and either fit it as an extra, or modify the taps at the sink. The outlet will need cutting through the floor, after making sure that bearers or chassis members are not in the way.