Remove the lenses from the plastic sunglasses. Bend the glasses to pop the lenses out if necessary.
Trace the shape of the lenses onto your transparency or acetate. You want two clear lens-shaped cutouts.
Color your cutouts. If colored paper wasn’t used to start with, then you need to use one red and one blue permanent marker to color your lenses. The left lens is colored red and the right lens is colored blue.
Place your new, colored lenses back into the frame of your sunglasses. Verify the red lens is on the left and the blue lens is on the right when wearing the glasses.
Place the glasses on your face. It’s important that the glasses fit well. Confirm you can see through them before watching something in 3-D. Adjust the glasses to fit you better if needed.