Would second the Cerebra guide and support from a professional involved with ds too. Our paed, GP and Headteacher were all very supportive when we applied. Whoever advised you to apply should be able to help you with the forms IME.
iv gone threw the Cerebra guide and spoke to the YISP lady that recommened it, she atm is currently arguing with CHAMS over my ds because of differant oppinions, 1 says he needs testing for ODD, and Asberghers while the other says no need. im at my ends tether and it is makeing me all so ill
Put in as much detail as you can and always overestimate all the time stuff. When I filled mine in I did it with my local Mencap office (Barnet in N London) and they were brilliantly helpful and we got a bigger award than expected. I would approach your local office and see if they can help. Good luck.