I had a spreadsheet with all the data, read all the Ofsteds and whatever I could find on online forums, I also had a checklist of what I wanted for my child taking into account her strengths and weaknesses. Then I went and visited the schools.
The visit was the most useful thing - I also had a couple of questions that I asked every single school and got a huge range of answers.
Some schools that looked fantastic on paper I really disliked when I looked round, others were the opposite.
You need to be very honest about the realities of your child. If organisation and executive function are not their strong point, will they be happier in a more liberal school where detentions are not handed out like smarties, or happier somewhere with a rather draconian structure where everyone understands the rules.
Is your child likely to be in the cohort that are aiming for a string of 9s at GCSE and dreaming of Oxbridge? If so, make sure the school has a decent cohort achieving that. Or are they more average? Maybe look for a school where the focus isn't all on the highest achievers and those at the bottom.
Does the school have a specialism that is in line with your child's interests (or one that they would hate?) If so, are there aptitude tests, specialist provision etc. Watch on that one as well - if they specialise in Sports, do only the scholars get to be in the teams or is there room for the keen but challenged?
What's the SEN provision like - both if you do and don't have a SEN child. If fantastic for SEN, then great if your child needs that help, but also give them a bit of slack if the grades aren't as shiny as the 'hopeless for SEN' school down the road.
I looked at over a dozen schools as we were targeting aptitude tests so not restricted to local area, and my ranking on the spreadsheet changed drastically after visits. One school, that I'd applied for the scholarship as I liked it so much on paper, we ran away from the open day it was so awful... but it's hugely competitive for places and other parents love it.