Draw out a design! I did this free hand onto the vest with pencil but you could also do it on paper/card and trace over it onto the material (if its thin/white).
Paint the design with a fabric paint... I used 'TULIP, Slick dimensional fabric paint'. It's quite cheap for a pack of colours on amazon and they work really well!
put the top on a flat surface and leave to dry, then outline the painted images with a black fabric marker (be careful its not one that spreads when washed!)