Start now getting as much medical evidence as you can to support your DD's claim, PIP does not take any notice of DLA, so be prepared to have to start from scratch with copies of reports, medical letters, consultants findings, assessments and anything else to show her specific needs.
Start now trying to keep a diary of how she is affected by her conditions every day and how much support she needs. It's good practise as you are asked to do so on the forms, this way you have more notice and can spot things you may have missed if you leave it until the last minute.
Apply to be her appointee.
Join Benefits and Work so you have their guides to fill in the forms and see the CAB website. www.citizensadvice.org.uk/benefits/sick-or-disabled-people-and-carers/pip/help-with-your-claim/fill-in-form/
People have posted about FB groups being very helpful.
Look through previous MN posts on PIP here and the main boards.
Be aware that some private advocacy groups ask for payment. Avoid.
Using guides and advice, complete and submit the forms.
See whether it's a face to face or home visit or telephone appointment for the ATOS or CAPITA assessment.
Make sure you request a copy of their assessor's report.
If necessary request Mandatory reconsideration
If necessary, go to Tribunal.
Always keep copies of everything you send to DWP and their responses.
Always send correspondence to DWP using the Post Office Tracked system, it's about £7 each time, but it stops them being able to say you missed a deadline or never sent anything, because the Post Office show a date and timestamp of it being received by DWP and give the signature of the person there who accepted it.
Oh, and Good Luck!