The best way to learn how to make them is to try to make them. It’s not difficult but it can be tedious:
How can I make a brush similar to the Stamp Heart?
Random Brushes, how do they work?
Other topics are available if you so a forum Search for “.gih”.
It would be quicker and easier to try the ‘Opacity dips’ technique with Fuzzy Dab or Distance with Repeat.