On doing it well rather than merely doing it, four things that matter:
Anchor text should describe the destination. Not click here, not this post: the words a person would use for what is on the other end. This is the part that carries meaning, and generic anchors waste the link entirely.
Vary it naturally. The same exact phrase every time looks manufactured. Write the sentence and let the link fall where it fits.
Place them where a reader would want them, which is usually mid-article at the moment a related idea comes up, not in a block at the bottom. Related posts modules at the end are fine and they do much less than an in-text link.
Link both ways when it makes sense. New article to old is the habit everybody has; going back and adding old to new is the half nobody does, and it is where the value is for a four year archive.
One more thing worth doing while you are in there: fix broken internal links and redirect chains as you go. Four years of a site almost always has some, and they waste the exact thing you are trying to create.
And build the habit going forward, every new article links to two or three existing ones and you add one link to it from somewhere older. Five minutes per post and you never have this problem again.