I used the Benefits & Work website for guidance. First time round I received standard rate PIP, and after a renewal I'm now on enhanced rate x2. I had a fairly lengthy phone assessments both times, but there was a lot to go through to demonstrate why I should be awarded points based on my needs - lots of aids, lots of prompting, lots of assistance with eg food prep, and lots of treatments/side effects, pain when walking etc.
Aids can include using your phone for lists and reminders, and assistance can include a friend or relative phoning you with reminders.
I have psoriatic arthritis, deteriorating osteo-arthritis, psoriasis and other auto-immune conditions, that are not very well controlled. I also keep a lot of files of paperwork related to it all, and have made sure I have online access to my GP records so that I print off what I need when I need to.
You have to stick to your guns in the assessments, I've found, especially about pain. I have pain all the time. That means every step I take is painful. That means that I cannot walk 'safely' as per the regulations, not even one step, let alone 20m. That means I qualify for douze points, even though on some days I could drag my arse to the postbox on the corner on my crutches if I had to.
Hope that helps. Good luck 