We did DISCO with CAMHS.
As needalife says, it kind of starts at the beginning and goes through, just about everything you could think of.
We started with my pregnancy, his birth, how he feed and slept as a baby. First milestones and developments. And then we went on and on and on. A diary of his life, basically.
So, here might be a good point for me to suggest to you, as you probably have already done, to make detailed notes, to take with you:
did he cry alot, breastfeed alot? colic? when did he walk, talk, did you have concerns, wehn, why. give the exact age he was, the exact date, if you can.
She kept asking me when he did a/b/c/ and I couldn't remmeber, so it might be a good idea to refer to your red book, as part of your notes, before you go.
Right down every date you can remember.. when you went on holiday and he said to the woman in the caravan next to you. 'Blimey aren't you fat', when he went round to your freinds house and said .....
the fact that your mum noticed a/b/c, and thta your friend said he was ' a-social'. and that he 'talked like a little old man'. That he was obsessional about .... pottermore, or thomas the tank. The time, aged 2.2 that he..... flapped and had a tantrum that lasted .... hrs etc etc.
You need to write down and remember every single that's every happened. every time you thought now thats odd.
You know how the triad works, right? so you know what they are lookign for, for a diagnosis ? right.
I'm only suggestign these things becasue its so easy to forget and kick yourself later, that you forgot to mention some of the most important things.
HTH