First off, get your materials together. You'll need some fabric (preferably fabric with no "right" side), batting, scissors, an iron and a sewing machine. You could technically do this by hand but it would take quite a while. With a sewing machine it took me about 45 minutes start to finish.
Start off by ironing your fabric to make sure there are no wrinkles that might cause a discrepancy in measurement.
Cut two strips of fabric and your batting 2x as long as an iPad plus two inches and as wide as an iPad plus two inches [Height: 9.56 inches (242.8 mm) Width: 7.47 inches (189.7 mm)]. Next, line up your two fabric strips on top of one another and then on top of the batting.
Sew all the way around the two long side and one short side leaving a bit of room before getting to the last short side. On the last short side you're going to fold a bit of the fabric back and sew it down so when you turn the thing inside out they're already stitched together. I hope that makes sense. I'm not sure how to explain it without being able to show you...
See how the edge in the next picture is folded back? I'm sewing just an inch on either side leaving an opening to turn it inside out...
Before you turn it inside out, trim the edges and cut the corners at a diagonal. This will help the corners look better when you flip it.
Once you have it turned inside out, pull out your iron and quickly press the edges. Be careful not to get your iron too hot or you'll melt the batting. At this point you should have a long strip of slightly squishy fabric.
Now we're going to do some quilting. I wanted to do diamonds but you could do squares or triangles. Just sew through all your layers in whatever design you'd like.
Sew all the way around your fabric to create a nice edge. Be especially careful to sew shut the open end from earlier! Once you've done that, all you have to do is sew two long sides and one short side together to create your pocket and you're done!
You could also flip your cover inside out if you don't like the exposed seams, just make sure to break out the iron and press the edges for a cleaner look.