Ask a cast member whether paintbrushes have been hidden on the island on the day of your visit. The hunt isn't always done and you don't want to waste time searching needlessly. Also, only 10 brushes are hidden, so you'll need to be on one of the first rafts of the morning to have a shot at finding one.
Search the island strategically, involving everyone in the family. Adults can look inside holes in the caves, atop shelves and in other high areas. Keep it fun and don't forget to enjoy the island as you hunt.
Stop looking after you find one brush to keep it fair for other families. You can only claim one prize per family, so put any extras back where another group can find them.
Turn the paintbrush in to a cast member and claim your reward. The prize may vary, but it is often a Fast Pass for every member in the group to ride Splash Mountain or Big Thunder Mountain Railroad.