How to get signature racewear in Umamusume Pretty Derby?

In Umamusume: Pretty Derby, trainers can unlock signature racewear for their horse girls by increasing their star rating to 3-stars. This is achieved by acquiring Star Pieces, primarily through scouting duplicates and exchanging Goddess Statues. Upgrading from lower star levels requires a significant investment of Star Pieces, with 150 needed to reach the coveted 3-star rank, unlocking the unique outfit.