The problem is, there is no definitive guide, "Do that, get merit". Basically they're telling you,
"go ahead, come up with your own idea how to get merit on our site". You cannot even buy it directly, only informally. I believe they could at least make certain manageable tasks so that people could get merit for doing something.
Yes, I earn merit by helping to moderate a few topic of the Russian section of the forum.
But you are not telling how you became a moderator.
There is no standard or definitive guide because merit is in the hands of people who have it, so it will bound to their subjectivity and point of view for some matters and things. So even though you are making a high quality post, it is still up to the other person if they want to give you merit or not. Making a great posts will only increase the chance of you to get merit, with that being said, it is a better thing to have more chances to get merit because you made a great posts rather than not having any chances to get it.