Briefs in civil appeals
Once the appellate court files the record on appeal, you will have to prepare your brief. A "brief" is a party's written description of the facts in the case, the law that applies, and the party's argument about the issues on appeal.
The briefs are the single most important part of the appellate process
The record on appeal (the clerk's and reporter's transcripts or other form of the record) provides the appellate court with a picture of what occurred in the trial court. But it is the arguments in the briefs that explain whether or not there was a legal error in those proceedings and whether it changed the outcome of the case.
The best briefs contain your entire argument, guiding the appellate court through the case and using the record and legal authority to justify your points. Because of the specialized knowledge necessary for writing a good brief, the briefs are also by far the most difficult part of the appellate process.
Once you've prepared your briefs, you need to decide if you want also want to argue in person, called oral argument.