Sketch out a geometric design. Draw directly on the canvas. Use the straight edge. Keep each shape separate. Alternatively, allow them to overlap. There is no one correct way to execute this.
Tape around one of the shapes. Leave the shape itself exposed. Use the painter's tape. Rub the tape down well.
Paint over the tape's edges with matte medium. Use the medium directly from the container. Do not dilute it. You just need a thin layer at the seam of tape and canvas. This prevents seeping. Allow the medium to dry. You may tape another shape while waiting.
Paint in your shape. Using a single color is bold.
Pull up the tape.
Repeat for other shapes.
When the paint within the shapes is dry, tape off the spaces between them. Paint them using similar method.