The mechanism is simple enough to use without understanding it deeply. You add fields describing the source, the medium, and the specific campaign, and analytics reads them and files the visit accordingly. Free builders will assemble the link for you, and the whole exercise takes under a minute per link. What matters more than the mechanics is being consistent, because the values are recorded exactly as typed and a source recorded three different ways will appear as three different sources.
Agree your naming before you start rather than after, because retrospective cleanup is not possible. Decide whether it is newsletter or email, lowercase or capitalised, and write the convention down somewhere you will find it in six months. This sounds trivial and it is the difference between a report you can read and one showing eleven variations of the same channel that nobody can total.
The places worth tagging are the ones you control and would otherwise lose. Links in email campaigns, links in social posts, printed codes, links given to a partner or referral source, and anything in an advertisement not already tracked automatically. Do not tag internal links between pages of your own site, which is a common mistake and corrupts the data by restarting sessions and reassigning the original source.
The payoff arrives in about three months, when you can answer which specific email, post, or partner produced visitors who actually did something. That answer changes where you spend the following quarter, and it is unavailable to a business whose links were untagged, because the information was never captured and cannot be reconstructed afterward.
Keep the tagged links somewhere you can find them again, such as a simple sheet listing the link, where it was used, and the date. Rebuilding a link six months later with slightly different values fragments the reporting for the same campaign, and the record also tells you where a link was placed when you later wonder why traffic from a source stopped. It takes seconds per link and it is the difference between a system and a habit.