The first step in becoming a Driving Instructor is to Apply to the ADI (Approved Driving Instructor) register which is governed by the DVSA (Driving & Vehicle Standards Agency)
You can do this online at…
https://www.gov.uk/become-a-driving-instructor/
You must be at least 21 years of age and have held a licence for 3 years.
You must go through a criminal record check to ensure you are a fit and proper person. Once your CRB check has come back successfully you are then free to Apply to the ADI register. Again this is done online and you will be given a link to log on. You will then be given a PRN (personal Reference Number).
When you have your PRN you can go on to book your Part 1 Test (theory & hazard perception)
This is all very simple and can be done in about 4 weeks with the CRB check being the only problematic part. Depending on various aspects such as if you have had one done before or not, or if there are any concerns with the check.
More detailed information can be found on the government website…
https://www.gov.uk/guidance/guide-to-the-approved-driving-instructor-register/