I did it many years ago, when I was a parent helper at my local primary school.
It was the middle of winter and snowing heavily. It was home time and parents and carers had been allowed into the school to shelter.
An elderly man suddenly keeled over, I helped the class teacher to lay him on the floor then she went to call for an ambulance leaving me with the man, several panicking parents and 30 small children.
The man suddenly stopped breathing, so I started mouth to mouth whilst another mum did compressions.
One of the parents of a child in another class was a GP, so somebody went to find her.
By the time she arrived, we had got him breathing again. So I gladly let her take over, collected my own child and headed home.
On my way, I could see the ambulance struggling to get up the steep hill to the school. A large group of parents were pushing it up the slope.
I knew roughly what to do as my brother had been in the same situation 3 weeks earlier when his father in law suffered a heart attack. I'd asked my brother what he did and some of it seemed to have stuck.
Sadly the man died later en route to the hospital.
As a consequence, the GP, who admitted that it was the first time she had ever done CPR, arranged for a training session for all the staff and volunteers at the school.
Since then, I have done several more CPR courses.
Non of them helped me when I found my husband dead, but still warm three years ago.
The call handler talked me through it and together with my daughter, we worked on him for 20 minutes until the paramedics arrived.
They tried for another 20 minutes but it was too late.