You're in the UK right? Get a profile on musicteachers.co.uk with some audio samples. Post a couple of ads in the relevant parts of local music shops, post something on Facebook along the lines of "Guitar lessons available please enquire for details" (Since it's Christmas you could take advantage of that by putting it forward as a great present or something). Though to be honest, most of the students I've had have been through referrals rather than direct advertising, but at the start you'll probably want to do as much as you can to get the word out that you're available for lessons.
be real cheap

but seriously just let people know youre teaching
alot of my student come to me from other students and me from seeing my band play and such and i advetize on my bands page sometimes
you could set aside some teaching time.
perhaps start with saturday mornings from 9.30-11.30
next you need to fill this time with students.
if you travel out you will find more work initially.

so lesson 1 = 9.30-10.15
lesson 2 = 10.45 -11.30

in regards to charging at first you will be learning on the job so i suggest you start by charging a minimum that you need to get started.
the bulk of lessons will be children , so you should make contact with as many parents as you can.
building real life relationships will give more sustainable income for your business.
perhaps you can start with your neighbours on your road. just do a morning's canvassing. knock on each door and explain what you offer?

you could also offer to give a demonstration at a local school.
perhaps in assembly or after school ?
most schools put on a christmas play , but next year they will have individual class assembly. you could write a letter to the school offering to help with the music next year, if you do this for free you will get good publicity with the key market!