Learn the scale (i.e. D minor scale.) Fill it out in the Cascade Method theory book.
Fill out the chord progressions for that minor scale.
Begin writing music!! Use any note from the (D minor) scale, as well as any chords from the chord progression.
You might start with writing mini patterns that you feel comfortable with and then put your hands together. I would suggest starting by keeping your left hand super simple until you feel ready to do a different rhythm.
i-iv-v-i would be the most obvious chord progression. This is a great chord progression to memorize as a starting point for composition.
If you are playing chords with your left hand and you’re worried if it will sound good or not, you’re always safe playing the same chords in your right hand. If you are holding D minor in your left, you can play D minor notes in the right hand. Remember to always stay on the scale for the best chances of it sounding good!
Remember to just let go and TRY! Don’t overthink it and have fun. As long as you are in the right scale or key, then there really are no mistakes. 
Film yourself! This way you can really get lost in exploring music. Then, when you listen back, you may pick up on a melody you might not have remembered.
Try a random chord progression! Write out a random order of chords and then play them. Do you like the way it sounds? Is there anything you want to change?