To decline an offer, contact the hiring manager via e-mail. State your appreciation for the offer of employment. State very clearly that you are declining the offer. You can offer a reason, but it is not required.
To accept an offer, call the hiring manager to verbally accept the offer, and follow up with a confirmation e-mail. As mentioned before, it is important to put everything in writing. Include a confirmation of your start date, position title, and rate of pay. Sign and send the required acceptance letters back to the employer per the instructions provided.
Once you have accepted an offer, do not continue to interview. Negotiate for more time if you are not ready to commit to this employer. It is unprofessional to accept an offer and then not honor you commitment.