I give this answer as per my experience:
• Narrate your story for all your responses to various pictures while practising at your home. Narrate by imagining that you are narrating in the real SSB interview. Keep an eye on time too. Imagine yourself sitting at various positions during narrations. Doing so your fear decreases and confidence increases. Confidence is directly proportional to practise.
• In SSB, after completing with PPDT story writing, candidates are generally given sometime before proceeding for Narration and GD. In this time narrate your own story as many times as possible in your own mind. Fine tune your points and have a clear cut story. Doing so your chances of fumbling reduces.
• During narration attentively listen to the narration of other candidates and this will help you in contributing points in GD.