You can do Royal Yachting Association (RYA) courses. The RYA is the national governing body for sailing, and sets standards for training instructors.
You'll need to be:
- over 16 years old
- physically fit and able to swim
- a competent, experienced sailor
You'll also need to pass a sailing assessment and do further study before you can start training as a sailing instructor with the RYA.
Courses are done at local sailing clubs or with private training companies.