1. Measure the space using a tape measure. For a snug fit, try to make your measurements as accurate as possible. Lay the cardboard on a pliable surface. Either a cutting mat, or your yard will do the trick. Don't try to cut your cardboard on your floor or concrete. With the pencil, mark the dimensions of your space on the cardboard. Since mine was rectangular I measured out the four corners and then used the yardstick to connect the dots with a straight line. Once you have the cardboard marked, cut it with the box cutter. Error on the side of cutting it too large. You can always cut more off.
Once the piece is cut, take it back to the area and make sure it fits the space. Adjust it as necessary. Don't worry about the edges being rough or slightly uneven. You are going to cover it with fabric.
2. Once the cardboard is cut and tested for fit you are ready to measure out the fabric. Lay the fabric out flat and mark the dimensions of the cardboard adding an extra 1-2 inches to fold over. I was not worried about lining up my pattern, but if you are pay special attention on how you cut the fabric. Be sure to cut your fabric on a cutting mat. Again, don't worry about straight lines. The edge of the fabric will be on the bottom side.
To make the corners easier to lay flat cut two diagonal slits in the fabric on each side of the corner.
3. For the final step, lay the fabric top side down on a table. Lay the cardboard bottom side up on top of the fabric. The only part of the fabric you should see is the border that is going to be folded over. Using the glue gun, start to fold over the fabric and glue as you go. Be careful not to pull too tight. When you get to the corner, glue the center section down first and then fold and glue each of the side pieces last like in the photo above. Once you've finished gluing all sides down allow the glue to dry.
You're finished! Lay your completed piece in place and start using it.