Make sure that you can legally teach people to drive.
If people are paying you for lessons then you must be on DVSA's Register of Approved Driving Instructors (ADIs). Even if they are just paying for the petrol you use, you will be breaking the law if you are not an ADI.
Make sure your driving licence is valid, and make sure you report any motoring or non-motoring cautions or convictions to DVSA – these may affect your licence to teach.
On the road
Display your ADI certificate in the passenger-side edge of the front windscreen when you’re teaching – this is a legal requirement.