The basic premise or formula is to recognize that when your skills + traits + behaviors are placed in the right environment the outcome will give you a greater chance of success and happiness in your work.
You may have received a job offer that is not what you hoped for. An attempt at negotiating the sticking points went unresolved and you have made the decision not to take the position. This doesn’t mean you never want to work for that company in the future so be graceful and professional.
Do not delay in telling them. It is your choice to call or email based on how you have communicated with them in the past. You may need to tell a few people but do it right away. They have other candidates to call once you decline.
Give them a reason. Is it the offer? Decided not to relocate? Taking a position with a different company? If you may work for them in the future, let them know you think highly of them and possibly in the future things could be worked out. If you are clear as the reason it gives them a chance to address the concern, and you never know.
Did you accept the position and now want to get out of it? You need to be very certain of that decision as there is no going back after you back out of a promise. If you signed a job offer letter, look at the legal repercussions that may be in the language on the contract. If you have any questions regarding the contract, you may want to check with a lawyer so that you are aware of potential consequences.
You will want to keep on the best possible terms once this offer is rejected. Express gratitude for their time, compliment them on their professionalism, be brief and try to do it on a call versus email.
For a deeper dive on what to say check out: https:/www.themuse.com/advice/how-to-gracefully-turn-down-a-job-offer
Want a marble in your jar? Your homework is to reflect on how you got so far down the road with this company and then changed your mind. Try to learn from this event, so that it can be avoided in the future.