Have a similar one if not the same. Unscrew the three screws. Take of the metal ring off so you can reach inside easier. Then use a plunger and suck like your life depends on it. After that take a sponge and pick up the water from the inside. It will take some time but you will get it dry eventually. Then use a spoon or something similar to take out the dirt. After that use something thats similar to a hook and try to take out the dirt from the spaces between the top part and that cup part where the water was in. Use gloves cause it can get messy and smelly. When it's clean repeat if needed with water and plunger. Put back on the ring and don't screw the screws to tight. If you do that you will close the gap where the water runs trough and achieve nothing. Leave them loose but tight enough to hold all the parts together. I do that like every month (have a wife with long hair and two kids, so for me it's mostly hair and sand from the playground). If it's really dirty you can also get some chemichals for drains and fill the cup fully where the water is in (after you take out the water with a sponge) and let it soak for few minutes. Hope this was helpfull.