Think about the story being told. It is very important to visually convey the narrative. Figuring out the overall theme of the story is a good starting point. A story about love looks much different than a story of rage and murder.
Analyze the characters. Think about the nature of the character, the social class, age, occupation and everything else. All of these things need to be figured out by the audience quickly, and it is the job of the costumes to aid in that. Dressing a servant in fine clothes or an old witch in virginal white gown does not make sense and confuses the audience.
Study historical period. Opera is almost always performed in historical costumes. Many periods are very complicated and require many pieces such as corsets and bodices, wigs, three-piece suits and more. Sometimes these things can be faked in clever ways, but most operas take much money, time and technical skill to produce.
Make bold choices. Opera is known for being lavish, luxurious and often over-the-top visually. The costumes be equally dramatic. Choose bold colors and lots of flash.
Consider movement of the singers. While opera costumes are often heavy and restrictive, consider exactly what the singers need to do physically. There may be a scene in which a character falls to his knees or raises her arms to the sky. The costumes need to accommodate these things and not restrict.
Remember the set and lights and how they effect the costumes. Considering the set is especially important, since lights can be changed up until last minute. The colors and aesthetics of the set should be harmonious with the costumes, not fighting each other.