apparently unwritten map making logic for standard osu! is this:
the harder and hardest maps are the easiest and most fun to map
the normal and easier maps are the hardest and the least fun to map. in general for starting mappers its recommended to start with "hard" or slightly harder difficulties, so around 4*-5*, although i dont know if the same applies if you yourself arent at that level yet.
i think it's because you don't have a lot of restrictions placed on you regarding flow and placement; you're more free to pack the map full with whatever you want, whereas on easier difficulties you'd have to "under"-map a lot in order to make it easier to play.
general rules apply to making a map playable and enjoyable. the most important thing is to make it readable and logical - one should generally be able to predict or guess where the next circle will appear based on previous placements or slider orientation, use sensible spacing (although this one can be bent a little), etc. i think the rest will follow from that.