What I do is I would look at my whole list of niche subreddits and find comments made by other people that get a lot of upvotes.
I would then use the same ideas that they have and may apply my copywriting magic to the content (and basically post the same stuff but on different subreddits in the same niche) to get karma.
This works 50% of the time because it turns out that these subreddits, although they are within the same niche, don't exactly have the same audience.
Some audiences have different preferences, so this technique can work only 50% of the time.
With that said, if you're just starting off with karma farming, try to do it with images and videos.
That is easier to pull off.