Do well on ALL of your 3rd year rotations. For now try to figure out where you want to rotate based on location, advice from upperclassman, places where you have the best chance to match, etc. Then in december e-mail the the programs to set up rotations for next year...by then you should have your school's 4th year schedule as well. I would try to do as many 4 week rotations as possible to increase your chances of matching as a lot of DO programs don't interview/seriously consider people who did not rotate with them (there are exceptions).
Once you have your audition rotations set up, study and rock step 2! Before you start your audition rotations and AFTER step 2, get netter's concise orthopedic anatomy and handbook of fractures. Get very familiar with hip, distal radius, ankle, and proximal humerus fractures and their treatment/classification. Learn how to read x-rays, what to look for, how to describe fractures.
E-mail people who have matched/upperclassman on how to excel on ortho rotations, where they rotated/interviewed/matched, what those programs look for, how they prepared for those rotations, etc.