Trimming wings can be tricky. One hint is to gently grab your bird and wrap him in a towel. Since birds don't willingly hold out their wings to let your trim them, make sure you are able to get to a wing and keep the rest of the body snugly in a towel in case the bird fights. Gently extend the wing. The primary feathers are the long feathers. For cosmetic reasons, leave the first two or three primary feathers alone. Trim the next five or six feathers to just below the level of the primary coverts and inline with the coverts.
For more suggestions, read How to Trim Your Bird's Wings