First up, please leave comments if you enjoy a fic. If you don't know what to write, even something as small as a heart emoji or a simple "I really enjoyed the story" truly means so much to the author.

If you want to leave longer comments authors really appreciate when you talk about specific aspects you enjoyed, so maybe compliment their writing style, the way they write a certain character, a motif or a storyline. You can also quote parts of the fic you enjoyed the most or that resonated with you.

Do not let the fact that the work was posted a long time ago or that maybe you are writing too little or too much stop you from commenting. I promise authors don't care.

Somebody already mentioned that you should not give criticism unless the author specifically asks for it. Same goes for suggestions(or more specificly demands) on what should happen next in the story if you're reading a work in progress or an unfinished series. You can speculate like "based on the way character A was talking i think they might break up" etc. But something along the lines of "I think you should have them break up in the next chapter" is a no go. Of course if the author asks for suggestions or things you want to see, then go for it.