I started off teaching in two primary schools. I hired their hall after school and asked the secretary to send a letter to every studentintroducing myself.
A week later I collected the reply slips. I had far too many responses to teach all of them alone so man I trained with assisted until numbers were more manageable (if you do not have anyone to help, take a few at a time).
The drop out rate is tremendous for white belts, so it doesn't take long to become manageable.
At the beginning of each year I readvertised.
When I had sufficient graded people who wanted to train more than once a week I opened an evening class and a few from each class attended.
Later, parents joined in as well. Once there were a couple, others followed.
This was 6 years ago, I now have men, women and children. I still teach in two schools and also Tuesday evenings in the local community centre and Saturday mornings. I have around 75 students and a waiting list of around 20.
Obviously, when you aproach the school, they will want to see your qualifications, insurance documents and current first aid certificat. Most will also want a risk assesment carried out by you and in the UK, a police check is also required.
If you would like to discuss how I aproached the schools in more detail, feel free to email me firstname.lastname@example.org