Measure the door opening's height and width. Add 3 inches to the total width.
Purchase a heavy-duty fabric to use as your divider. Avoid thin materials that will tear easily.
Measure out the length of fabric you need and cut it with scissors. Cut this panel in half lengthwise to create two individual panels.
Fold over the top 3 inches of each panel, and sew them in place to create a pocket to run the curtain rod through. This is a simple straight horizontal stitch from one edge of the panel.
Cover the two long sides and the bottom of each panel with fabric binding. The binding can be the same color as your fabric or something that makes it stand out.
Slip the curtain rod through both panels and lift it into the open doorway. Adjust the springs on the curtain rod so there is enough tension that the fabric panels won't be knocked down when someone walks through them.