ITs easy but you need to have a much bigger wish list, which you then break down into nice-to-haves and must-haves. Then you need to think about what their wish list might be and how many boxes you can tick.
EG. You need a driver. That means you need someone over 21 (otherwise you will struggle with car insurance- up to £5k extra with some insurers if under 21). If you need someone to help with homework they need to be not just fluent in English, but a graduate too.
So with all of those must-haves you might need to be flexible on other things. Do they really need to be able to cook reasonably well? Or just be able to rustle up an omelette and cook some sausage and mash? Do you need a girl or could you have a boy instead (will make a huge difference in the quality of applicants you get, as there are tons of male APs looking for jobs, but good girls tend to have lots more choice open to them). How long do you want them to stay for? 6 months appeals to people dong stuff before getting on with their real jobs, longer appeals to those who don't really have a life plan yet. There are pros and cons to having people stay for longer vs shorter periods of time (not least the headache of replacement) so have a think about that. Also, do you want them to be a 'mate' to your DCs, or a pseudo-parent? That will affect the CVs you should be looking at.
Then there's things you need to think about to make your job more appealing. Can you offer gym membership? use of car? Oyster card? Are you going to give them a double or single bed? Will you let their friends visit and stay over? When they're off duty is there a separate sky TV so they can watch their stuff without it bothering the rest of you (think America's Next top model for girls, UEFA footie matches for the boys). If not, how flexible will you need to be yourselves?
IME agents are great at finding you young girls with driving licences and reasonable English who are not long out of school and looking for a life experience. Most of them (again, IME) haven't lived away from home before and that is primarily why I personally don't use them - I don't want to run the risk of hiring someone who expects to be 'parented' themselves. Using Greataupair or aupair-world is a hassle, but means I can sift out the unsuitable ones easily. If you do the DIY route bin ALL applicants without a European passport or some other right to reside (some commonwealth countries are ok). - Don't even look at the profiles of non EU non commonwealth, because you will not be able to hire them except on student visas which will be too restrictive for what you are looking for.
On the plus side, having au pairs around isn't that huge a shift, and my DCs have loved it - always someone there to play on trampoline or have a game of chess with or kick ball round the garden. There are lots of really nice, really normal young adults out there, and most of them make your lives soooo much easier (I have mine doing the family shop, taking pets to the vet, sorting out deliveries and suchlike. Bliss)