Decline quickly, thank them properly, and leave the door open. Fill in the boxes and the email writes itself.
Filled in with an example. Type in the boxes to make it yours.
Subject: Business Analyst offer
Hi Rachel,
Thank you for offering me the Business Analyst position, and for the time you and the team spent with me through the process.
After careful thought I have decided not to accept the offer, as I have accepted a role that is a closer fit for where I want to take my career.
I enjoyed meeting you and learning about the team's plans for the new platform. I would be glad to stay in touch, and I hope our paths cross again.
Kind regards,
Sam Hollis
Say no as soon as you have decided: they may have a second candidate waiting. A phone call first, then this email, is the most courteous way if you have spoken to them often.
If you had already accepted, this is a different conversation: read the article below before you send anything.
More on this: How to decline a job offer politely (with email templates)
Keep every letter with the job it belongs to: jobtracker.co.nz is free while you're between jobs. See all the letter templates.