The best way to prepare any child for moving from primary to secondary school, is to start early in year six, think about the problems which may arise and work out strategies for your child to overcome them.
The Foundation for People with Learning Disabilities has produced various leaflets and the one for pupils entitled What's it about? is full of sensible, practical advice and can be downloaded for free.
There is also a free to download guide for busy families which again I would endorse.
The transitiion from primary to secondary school can seem daunting for any child but for one with Special Educational Needs (SEN) is can be particularly stressful. To minimise stressful situations, plan in advance as suggested and practise skills that your child may need for instance travelling on public transport to school.