How do I decline a job offer politely?

Reply promptly, thank them sincerely, state your decision clearly and keep the door open. You do not need to give a detailed reason.

Example:
"Dear Ms Torres,

Thank you very much for offering me the position of Laboratory Supervisor. After careful consideration, I have decided to decline the offer, as I have accepted a role that more closely matches my current career plans.

I really appreciated the time you and the team spent with me, and I hope our paths cross again.

Kind regards,
Ravi Nair"

Decline by email or phone followed by email, so there is a written record. Do it as soon as you have decided, so the employer can offer the role to someone else.

If you built a good relationship with the hiring manager, a short phone call before the email is a kind touch. Keep the call brief, then send the written confirmation. Do not ignore an offer or let the deadline pass without replying; silence leaves a poor impression and may be remembered if you apply again.