1. Buy metal screens at hardware store.
2. Store them against a wall in your foyer for 1 year.
3. Eventually try painting them white. When the metal still shows through the first coat of paint, forget about them and give up.
4. A year later, get motivated and paint a second coat. Store against a wall in another room for 9 more months when your wife says, "Don't put those back in the foyer."
5. Walk past them them every day and occasionally think what it would be like to actually install them.
6. After 3 more months, place the large wooden radiator cover on its side on the living room floor, remove the nails holding the old, warped metal screens, remove the old screens, cut the new ones to size and prepare to nail them in.
7. Realize you don't have any nails. Think, "Dammit, I knew this would take more than an hour." Go to hardware store and buy some. Come home. Misplace the hammer you had literally 30 minutes ago. Curse. Finally find it on the floor under the radiator cover. Apologize to your kids for cursing. Nail in the new screens and place the finished product back over the radiator. Brag to your wife about your handyman skills.
Note that your actual completion time may be longer or shorter than 3 years.