If you are clear that this shidduch is not going to work out, it is best to bring it to a close without delay. Going on another few dates, or even keeping the other person on a date for another couple of hours “in order to be nice” is inappropriate.
If you have only met a few times, it is okay to deliver the message via a messenger. But if you have met a good number of times, it is appropriate to handle it yourself. Someone has taken the risk of opening up to you; there is a duty of care on your part to lessen the impact by going about this with sensitivity.
It is generally nicer if you give a reason for your decision. This allows the other person to learn something from the experience, and it avoids them drawing unfounded conclusions. It is best to keep your remarks brief. Stick to what you planned to say, and be careful not to get drawn into saying anything you may later regret.
Assure the other person that you will maintain their privacy. Convey your respect and appreciation, and sincerely wish them the best for the future. Do not leave the situation open-ended. Don’t say something that gives the false impression that you may revisit it.