Rejecting a job offer is never easy. After all, this is a company you most likely already had multiple interview rounds with and someone who you could have imagined as an employer, so you do not want to burn any bridges.
Here a quick overview of things to do:
Even a rejection can be done well. It may be good to maintain a friendly relationship with the HR department of interesting companies for your future career.
If you cannot reach him or her, then try again, instead of leaving a message with an assistant. It is a sign of respect to "break up" in person, no matter if in personal or professional relationships.
After all, you as the applicant would like to know the company's feedback as early as possible as well. This will give the company time to contact other applicants and to continue the search without further delay.
Give your honest and pragmatic reasons of why you have decided to reject the job offer. It gives clarity on both sides and you will be remembered fondly.
Even if in the end you chose not to take the job, thank your interviewing partner for their time and consideration. It may be just a short line in the email or phone call at the end, but it leaves a positive impression.