Include how previous experiences and activities have given you extra skills such as leadership, responsibility, etc. Outline how they benefit you and how you can utilise them at uni. Assuming you have the substance, try to keep it as recent as possible (eg not before GCSEs). For example, any responsibility you’ve held in a job, school council, bullying mentor or a volunteering position rather than you once collected the glue sticks in at the end of Brownies.