If you start from scratch, factor it in when deciding on a cms, platform or site generator. I think RSS/atom owes a lot to WordPress, which has it enabled by default since ages.
When you want to add it retro-actively, the specifications are relatively straight forward and don't have many mandatory properties. RSS is a tad simple than Atom, but I'd probably rather go for Atom if I were to start over.
Also, JSON Feed [0] is still better than nothing, many of the major RSS readers quietly support it, and does have the small advantage that JSON is much easier to work with in more platforms than XML today.
When you want to add it retro-actively, the specifications are relatively straight forward and don't have many mandatory properties. RSS is a tad simple than Atom, but I'd probably rather go for Atom if I were to start over.
https://www.rssboard.org/rss-specification
https://validator.w3.org/feed/docs/atom.html
In any case: add a link rel=feed to the header of every page to enable discovery (https://www.rssboard.org/rss-autodiscovery - but works regardless of the format)