Cotton 14x33 cm.
Fold the longest site in two.
Then sew it together leaving a gap at the small site. You need a gap so that you can turn it inside out later on.
First you have to put the seam down and go to the edge. Now you sew the edge over 1,5 cm.. As shown on the next picture.
At the right edge you can see how to do that.
When your sure that everything is correct you cut the edges.
Turn inside down and make a small hem all around the mattress.
Now you can stuff the mattress.
Close the gap by hand.