The most common is the DBH Collective store. You can Apply For A Store and if approved upload your artwork to sell on apparel, phone cases and art prints.
The second method is to wait for DBH to put on a Special Contest. Special Contests are periodically setup with partners to allow any artist to upload their artwork for voting and the best artwork is chosen. You can see Past Contests using the contests drop down menu. You can also leave your email address on the Submission Page to be notified of upcoming contests.