Where to find one:
Nanny Agency, online job sites like NannyJob.co.uk, Childcare.co.uk
Local areas of parenting sites perhaps.
What to do:
Create a job description. Have a good idea of what the job will really involve, what you want them to be doing. The hours of work, the days of work, if you need someone who drives, if you need someone who will use their own car or if you will provide a car, what housework duties there may be (keep it quite minimal, if you need a cleaner employ a cleaner).
Set up payroll - do you know how to do PAYE or do you feel it's worth paying someone else to do that? If the latter, then look at nanny payroll companies such as PAYEforNannies.co.uk who can provide monthly payslips and assist with employer returns, reminding you when to pay HMRC and how much to pay.
Work out your costs - do not under estimate the costs involved. Use a payroll calculator like www.mranchovy.com/calc to get a feel for what Employers NI will be based on a particular gross salary. Work out the typical amount of money needed for daily activities. Consider cost of food (nannies will often eat with the children whilst on duty) and mileage (if nanny is using their car) or car running costs if you are providing a car. Cost calculation for 5 day nanny.
Do you have plans for holidays you will go on over the next year? Get the calendar out and work out which days you will not need your nanny and thus will want them to take as part of their annual leave. How much choice will nanny get in when they can take annual leave... how much annual leave must they have as a minimum (5.6 weeks x hours per week, or if each day is same number of hours then 5.6 x days per week).