I laid a piece of tent mesh onto the window opening to start.
I got the mesh fabric from a tent repair place in Berkeley called Narain's on San Pablo. It was about $10/yd, but its really nice high quality stuff that all the good tents are made of. It's strong and doesn't rip. It is fabric, not the kind of mesh on screen doors, which is kind of metal.
I sewed the velcro strip on top of the mesh, to hold the mesh secure in place.
It was difficult working with the light and wispy mesh. It was hard sewing it onto the more stiff canvas without getting waves in the mesh. It took me a few hours to do one window, mainly because I spent a lot of time pinning the fabric in place, and fighting with the sewing machine! But the time and effort was worth it. I just kept thinking in my head: fresh air fresh air fresh air!!
After all the velcro strips were sewn, I cut the extra mesh.
The final step was to cut off all the extra mesh that stuck out past the velcro strips. Once I cut it away (with a seam ripper, which worked great!), the windows look very neat. I can't wait to test the new windows out!