From your Dashboard in the app, click "Log!" in the upper right corner of the screen, and choose a workout. From the bottom of the "Log Workout" view, tap the note icon. 

You will have the option to enter your body weight and workout notes. Enter your notes and then tap "Done."

Your workout notes will appear in your Journal and in the site Activity feed along with your workout stats.

To edit your workout notes, choose the workout from your Dashboard. Then, from the "Workout Summary" view, tap the note icon and edit your notes as needed. Select "Done" in the upper right corner of the screen when you are finished editing your workout notes, and they will be updated.