but generally, you go speak to a light vehicle engineer that has been approved by your roads and traffic authority.
they will chat to you about what you want to do, and charge you a few hundred bucks for telling you how it should be done so that they would approve it. (i.e. using correct grade bolts, correct materials/adapters for the seat rails, yada yada yada)
once you've done this according to how they want you to do it, they will inspect your handywork, and if they're happy with it they will write out a certificate to say that they have approved this modification.
There might be an additional fee for the certificate upon completion.