Start by measuring the part of the luggage rack that you'll want the canvas to cover. I used the straps that were already there as a guide for how wide and long to make mine. Then, cut a piece of canvas a little larger than the measurement (for seam allowance). I cut my canvases to 18x18 inches.
Next, for the two edges that will be shown, you'll want to finish the edges with a sewing machine by folding it twice and then stitching on the fold. The other two edges can be left unsewn, since they won't be seen once finished. So, once sewn, you'll be left with a piece of canvas that is somewhere around 17x18 inches, two ends are sewn and the other two are left as raw edges.
(optional) If desired, this would be the time to add canvas wax to protect the surface. Instructions are according to packaging. We skipped this step, but it's great for creating a more water and stain resistant surface. I have used the exact wax I linked to before, when I made my waxed canvas wine bags, and it works great / is super easy.
Wrap the canvas around the luggage rack and flip the luggage rack over. Staple the canvas to the luggage rack underneath, as shown in the photo. Do this on both sides of the luggage rack, all the way across the canvas. Flip it back over and it's ready to use.