Do you have an artist or group of artists whose styles really speak to you? What thematics would you like to emphasize aesthetically in your works? What parts of the body or elements of backgrounds do you obsess over in detail? Those are good starting guide points.
Just pick up a pencil or pen and let some loose lines come together, but keep all that stuff in mind. It'll get you somewhere, eventually.
Your main problem is the transition to a different style. You have done well with realistic, and now you're trying to stylize things into a more cartoony look, with inconsistent results. I would suggest looking at the work of other artists.
For example, pick 20 artists with each a very different style you like, and reproduce one of their pieces (you should keep these excercises to yourself though since you don't have copyrights). It will let your hand try different things. Maybe you'll find you like this way of drawing noses, and that kind of line thickness, and those kinds of proportions.
It's important though not to stick too closely to any one artist in particular. When you copy someone's style, you end up being "like that guy, except not quite as good" which is never good... Instead try to make your own style, by finding out what you like to draw from many different sources of inspiration.