Just the oft repeated advice - be brutally honest, tell it at its worst when/if you're not there to support, and don't hold back so include all the gory details! After reading mum's application I got a phone call to double check a couple of points and the lady who called said 'gosh I feel for everyone in this situation, it's all so sad for everyone' and somehow that made me feel better about the things I'd written.
Don't just tick the box - fill in the boxes with loads of information and really hammer home what your Mum can't do anymore. If your Mum has someone looking after her, there is space for them to put their views and I made sure I put Dad's comments, reiterating how much he had to do and how he was struggling (he is 80 and at the time of completing it was recovery from surgery).
Don't worry if you think you keep repeating yourself, I went with the view that too much information was better than too little. Good luck with it.