Measure the length and width of each bathroom fixture and fitting, such as the clothes hampers and bathtub, as if it is a rectangle. This will make it easier to place each item on a scale floor plan, which makes visualization and planning much easier. If purchasing new items for your bathroom, collect the footprint information from the manufacturer's website or brochure. Otherwise measure your existing fixtures using a tape measure.
Measure how far your radiators, fixed towel rails and doors jut into the room. The door especially is important to consider as you must be able to open it fully to use the bathroom. These non-moving items determine the usable space in the bathroom in which you will be able to lay out the other fixtures.
Remember that you need space around your bathroom items for them to be usable. While it is possible to put a sink right next to the bath as long as the sink faces along the bath, putting a toilet facing the bath without space for people to sit down will make the toilet unusable. Arrange your items on the floor plan so usable space is not restricted.
Plan your bathroom layout to scale. Draw your floor plan on graph paper to make it easier to visualize where everything will go, and keep everything drawn to the same scale, such as one square of graph paper being equal to 1 square foot in the bathroom, otherwise the measurements you made will not help you.