Hi Faye
You need to write a reflective account which talks about you supervisions and performance reviews with your manager. You could add copies of these to go with your account.
Start with the one, say, a year ago and talk about the identified strengths and areas for development, what development was discussed and agreed (personal development plan) and then relate how that plan was put into action, courses attended/training/further experience and how this improved your performance. Then move on to your next review and relate how your progress was reviewed and your development plan updated etc.
Talk about how you have been learning and gaining experience and how this has helped you in your job. Talk about unplanned learning opportunities and how you used them, how you have obtained feedback from others in order to improve your performance.
I ramble on.

Dave