I suggest completing an entire episode first, with proper monster placement (i.e., take out ALL the levels where it's just two little rooms then an exit switch if you haven't already)
If you have trouble with coop it might help to make some competitive maps (DM/Duel), I think making DM and duel maps were a good thing to do before getting deep into single player.
You should try to make single player maps first though, just to be sure you won't make maps that will have HOMs or break, if they're not great don't worry about it, it takes a long time to get good at it. Just keep making maps and remember what you did right and wrong from your last one and apply it.
Delete weak links in your pack as you find problems and replace them with better ones (the pickier and more observant you are, the better). Experiment, make mistakes, separate the good from the bad, keep the good and learn from the bad, and eventually you'll have something great.