DD was very very ill in the run up to Yr 13, and had been unable to do anything in the two weeks before the start of term which had been set aside for University choice and first draft PS. It was medical school so an early deadline and an important PS.
What I did, when she finally confessed (at midnight ) that the first draft was due in at school the following morning, was prepare bullet points containing what she had done and roughly ordered into three paras. She had already written a (really good) first para drawing on her on-going experience and putting it in the context of her interest in medicine, so my aim was simply to get her work experience, volunteering and extra curricular down on paper, so the school were aware and could guide her.
In normal circumstances I think I would have simply read through her first draft and pointed out any omissions, again leaving it to her school to help with style and format.
The other thing I did was to print out each of the four medical schools list of what they are looking for, "Community involvement" etc and suggest she check carefully after each draft that she had covered each requirement, evidenced it and ideally said what she had learnt from it. In retrospect, though probably not surprisingly, starting from University guidance helps a lot especially for those subjects where you are not expected to be a gifted writer. It becomes an exercise in ticking the boxes and by the time you have done this you have used up most of your word count.
My assumption was that a PS' would need to be scored and the starting point for scoring would be requirements and desirables listed in the guidance.
I wish I had done this for DS who was applying for economics, another subject where a PS can be important for sifting. He ended up being rejected by three Universities despite a 4 x A* prediction in suitable subjects, and a huge interest in economics. He too struggled, though I left it to the school and did not get involved. It is difficult for teenagers to write something so very important, especially when they do not enjoy writing. He now feels that his PS was the issue. Those Universities who read PS' thoroughly, provide very detailed guidance.
The only other input I had into DD's PS, and again mainly because she was still in pretty bad shape, was the day before the deadline and with the word count. Cue a disagreement with DH who though she should also smooth up some of the more angular text. Given she was attaching an Ed Psych report confirming she was dyslexic, I thought it was more important for her PS to sound authentic. The girl they interviewed would have been the same as the girl whose PS they read.