Written for Malachain: the essentials, without the noise.
The best single-day visits follow a loose shape rather than a schedule. Start at a market or a bakery where locals queue, walk towards the oldest part of town, and let the side streets decide the route. Stop whenever something looks interesting. A city reveals itself through its everyday places far more than through a list of ten sights.
Save the one thing you really want to see for the late afternoon, when the light is better and the crowds thinner, and finish with a long dinner somewhere you noticed earlier. Keep a note on your phone of the places you passed and liked; it becomes your own guide for the next visit and a much better recommendation for friends than any ranking.
The most useful step is usually the first one. Pick a single suggestion, try it for a couple of weeks, and then decide whether it is worth keeping. Small, steady improvements last longer than ambitious plans that are hard to maintain once everyday life gets busy again.
It also helps to talk the subject through with friends or colleagues who have tried something similar. Their experience points out the details that guides leave out: how long things really take, which costs were unexpected, and which shortcuts turned out to be a mistake.
There is more to learn on this subject, and the linked website goes into greater depth. Treat this article as an introduction rather than the final word, and check any important decision against up-to-date, reliable sources before relying on it.