It’s also worth raising the bathroom door threshold by about 5mm from the floor in case the room fills with water (if someone covers the shower drain with a towel, for example). This will keep the water contained.
Keep it neat. You can never have too much storage in a family bathroom. Shelves can look untidy very quickly so opt for cabinets with doors that can be closed to contain everything out of sight.
A scale plan of your space is a good place to start. Measure carefully as accurate dimensions are key and include features that are likely to affect the design, including windows, chimney breasts and doors.