Identify the characters. This practice goes all the way back to William Shakespeare: you may remember those character lists in the beginning of his plays. These are just like a character breakdown, albeit a simple one, showing who is involved in the play. Writing a list of characters is your first step in creating a character breakdown.
Outline the roles of each character in the screenplay. You don't need to include a lot of detail on this unless you want to, but putting in some information about their occupation and relation to each other is helpful.
Add physical characteristics for each character. This helps readers to see what you see for a character, and brings that character from abstract into something a producer can work with. Your character's physical i.d. can read like directions for a police sketch artist, or it can be a little more vague, but get some visual demographics into your character breakdown to make your characters more tangible.
Include some other demographic information. Age breakdowns are often useful, both in writing and troubleshooting, and so the reader can identify the character roles implicitly.
Include any alter egos or other accommodations for plot twists. This is often an unnecessary piece of your character breakdown, but on the other hand, if your characters will have more than one role in a screenplay, it's best to detail that in your casting documentation. This holds true for characters that reoccur in the screenplay at different life stages, as well as characters whose dual identity will be revealed near the end of the script.