Obtain canvas or an outdoor fabric from an online vendor or your local fabric store, making sure it's the appropriate size. Lay your original gazebo cover over the fabric. Use a thin-tipped marker to trace the edges on the fabric.
Take note of the measurements of the gazebo top if you don't have the original cover around. Mark these measurements onto the fabric. Add two inches to the width and length for overhang. Cut the fabric with fabric shears.
Fold the overhand into a one-inch hem all the way around the cover. Hold the hem down with sewing pins. Sew in the hem with a sewing machine.
Stitch a leather cord long enough to tie around the gazebo beams. Install the finished gazebo cover onto the gazebo. Make adjustments if the cover doesn't quite fit.