The most important skill for game designer is attention to details and ability to observe and understand mechanics of other games, understanding why it works or not. Modern free to play game design is heavily based on human psychology e.g. behaviorism. I would rather suggest reading some psychological studies in that area if you're interested in free to play.
Another good technique to learn game design is trying to replicate interesting mechanics, visual feedback, rewarding systems, retention mechanics etc from other games 1:1 just for the sake of understanding, how it works under the hood.