First, make sure you've taken out all the hair from the hairbrush
Then, get warm or hot water (I get hot instead, figured it'd *clean* it more) into the small bucket and dump your dirty hairbrush. Let it stay there for 5-10 mins.
After 10 minutes in the hot water, put your shampoo around the bucket or straight to the hairbrush, and scrub it with your old teethbrush.
After brushing the dirt out, wash it with water. Replace the old water with new water, and put your brush back in (make sure it is not dirty anymore). You may put a pinch of shampoo in it if u want.
After 10 minutes in the new water, wash it again until it is shampoo free. You may dry it with a towel or let it dry itself. Now your hairbrush is good as new!
Hope this helps! 😊 I reckon you clean your hairbrush once a month, if u use it a lot.