Measure the width and height of the gazebo sides.
Determine how much fabric you will need. For the width of the curtain panels, you will want to buy one and a half times the width of the openings on the sides of the gazebos, plus an inch on either side for the hem. You will likely have to sew three or more lengths of fabric together to create a panel wide enough. For the length, you will want the same as the height of the gazebo, plus an inch for a hem on the bottom and about 2 inches for the pocket at the top of the panel.
Choose the fabric. Look for fabrics intended for outdoors use which are water repellant, mildew resistant and fade resistant. Canvas is a good choice, as are duck and vinyl coated sheers.
Sew the curtains. Use a straight vertical seam to sew several lengths of fabric together. Hem the sides and bottom with a 1 inch hem. Stitch a 2 inch deep pocket along the top edge for the curtain rod, or deeper if you are using a thick curtain rod.
Install curtain rod or track on the top inside frame of the gazebo. Depending on how the gazebo is structured, you may be able to use an existing piece of the frame if it is only attached to other pieces of the gazebo at the corners.
Hang the curtains on the curtain rod or tracks.