Using a flat iron to curl your hair is a lot like using any tool for the first time. So don't try this before a high-stakes event without practicing first, or you might be disappointed by the result. Here's how to prevent the most common problems.
Bumpy hair: The hard edges of a straightener can produce bumpy hair if you're not careful. For a more even look, wrap your hair loosely around the plate, and don't use sections smaller than 1 inch wide. A straightener with wider plates works best for creating waves since narrow plates create more curves and more opportunity for bumps.
Uneven curls: A flat iron can't give you bouncy, perfectly proportioned ringlets. That's the entire point! But if the curls you get seem terribly uneven or oddly proportioned, the problem could be the way you're dividing your hair. Each section you wrap around the iron should be about the same size. Be sure also to wrap each section about as tightly as the next, or you'll end up with some very curly sections and some that are only loosely wavy.
Hair damage: Some heated tools dry out your hair, potentially causing it to break. To reduce the risk of hair damage, don't use a straightener on already-damaged hair, and never apply heat for more than 10-15 seconds. Invest in a high-quality straightener that applies well-controlled heat.