How could I do it? What would be the best way to do it?
You register a company, you apply for all the required licenses, you go to a bank and you open an account, the moment you receive confirmation from your bank or the payment processor of choice that the client has sent you the funds you send him the coins!
If you don't want to go big and have your own "exchange", just go to localbitcoin or local crypto or bisq and you can exchange coins for fiat there.