Clean your non-porous mounting surface thoroughly.
Press the flat base up to the surface.
Offer the metal part up to the flat base on the wall.
Twist the cup anti-clockwise until it is tight.
The bathroom can quickly become a messy place, with razors, toothpaste, and toothbrushes just laying around. As well as being unhygienic this is a real eyesore for guests who use your bathroom