This piece for Malachain collects the advice that holds up in practice.
The newsletters people keep opening are short, personal and regular. One useful idea, one piece of news and one thing to do, sent on the same day every fortnight from a named person, will outperform a long monthly round-up every time. Write the subject line last and make it a plain description of what is inside rather than a tease.
Grow the list slowly with people who asked to be on it. A sign-up on the receipt page, a card by the till or a line at the end of each email is enough. Watch which links get clicked and write more about those subjects. Remove people who have not opened anything in six months; a smaller list that reads you is worth far more than a large one that does not.
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.