I start by setting a timescale. If you send it in too early they have time to query it, so I time it so they only have time to check it through but not argue it too much before the renewal date comes up.
Next I fill in the 'constants', all the general information like personal details, phone numbers, professionals' contact details etc.
Then I start a Word document and retype all the section headings from the form into it. I NEVER write directly on to the form because I strongly believe that they psychologically bully you into making your answers fit the space they provide. If it takes four pages of A4 to describe your child's social difficulties then that's how much you should use or you can't give a clear picture.
I work on the sections one at a time using the Cerebra guide as a reference - can't recommend that highly enough. I try to do a bit every day, even if it's just one section, otherwise it drops out of my head and I forget where I was up to. I also make sure I work on it in the mornings because that's when I'm most clear-headed. Green and Black's hot chocolate helps.
After that I bullet point key issues at the top of each section, highlighting main difficulties and what's necessary to address them. I also go back through the paragraphs and highlight (again, in Word) key words for ease of reference for the assessor.
I include reference to additional documents I'm enclosing with the form, which I number clearly. I also write a covering letter explaining how I've filled in the form and why.
I then fill in the frequency parts on the form at the bottom of each section. Usually I say 'throughout' for how many times a day and 'as long as necessary' for duration.
Finally I take a photocopy and send it registered post to make sure it's trackable. I phone DWP after a week or so to make sure they've got it.
The last time it took me about four months in total, but at least the claim was approved first time and I didn't have to go to reconsideration like I did the first time. It's time well spent if it means you don't have to do it twice, or even three if you have to go to appeal.
Great idea to get CAB to help if they can fit you in. In my area they are so busy they don't do DLA claims very often, although Crossroads Carers have DLA reps if you have a branch near you.