Begin drawing or lettering whatever images or words you'd like on your cassette cover insert that came with the case. Using the cassette cover insert that came with the tape is a great way to go so you don't have to worry about having your artwork fit the measurements of the cassette cover.
Color in or outline your previous drawings with markers. Coloring in the background around your drawing can be an interesting aesthetic in addition to outlining your drawings in black which will set off the color of your cover background.
Cut out some paper that fits the cassette cover if you decide to not use the fitted insert cover. Decorate using a mix of magazine cutouts, pencil and marker drawings and background colorings for an interesting collage effect. Another interesting visual option is to draw sketches depicting your favorite musicians on the tape. Trace your musician sketches with the glue stick and sprinkle with loose glitter. Shake the glitter off when finished.