Study the Trademark Trial and Appeal Board's Manual of Procedure (link in Resources). The manual will provide you with the relevant information needed to file pleadings and initiate other potential actions prior to filing your answer with the USPTO.
Obtain a copy of the trademark opposition you intend to answer. When the opposition was filed, you should have been served with a copy of the filing by mail. Study the document to determine the other party's basis for contention. Make note of the assigned opposition number, which will be in the form of “91xxxxxx.”
Draft your answer to the trademark opposition. In your answer, you must admit or deny every allegation asserted in the opposition. Assert in plain terms any applicable defenses to the allegations set forth in the opposition. Typical defenses include unclean hands, laches, estoppel, acquiescence, fraud, mistake, prior registration or any other common affirmative defense. You may also assert counterclaims in your answer when applicable.
Sign your answer and attach any exhibits as well as a certificate of service. The certificate of service is a signed pleading that verifies you have served a copy of your answer on opposing counsel. Serve a copy on opposing counsel by first class mail on the date specified in your certificate of service.
Navigate to the USPTO's Electronic System for Trademark Trials and Appeals. Go to the "File Documents in a Board Proceeding" section, and from the drop-down menu, select "Opposition, Cancellation or Concurrent Use." Type in your opposition number and click "Start."
Select the party that you represent from the identification menu and click "Next." You will be taken to the "Identify Paper" menu. Once there, select "Answer" under "Pleadings and Related Motions" and click "Next."
Upload your answer to the USPTO by browsing for the document that contains your answer. Note that the USPTO prefers all documents be submitted in PDF format.