I don't think there are any manuals to know what to do. But, on the other hand, I don't think either you need lots of years to reach a good level. Of course, you never stop studying music.
Perhaps, in your case, you should focus on the scales of each chord. That includes what notes are chord notes, what are tensions, and what are avoid notes.
You start with major mode and its seven chords-scales. In melody, to be sure, you put chord tones and tensions in strong places, and every note you want, including avoid notes and crhomatic ones in weak places.
What are strong points? The beginning of the measure, an also the beginning of every division of the measure. Also, a tone followed by leap or by silence becomes a strong note...
Well, there are many things about this. And besides, when you know the essentials, you can break the rules and make other things supposedly "wrong".
Being self-taught is hard, many times you don't know if you are right or wrong, if you don't have anybody to tell you. But being self-taught is also a challenge.