I use white solid fabric for my labels. The size can vary but I tend to make them big as a souvenir card if the quilt is a big one.
Cut out a piece of fabric slightly bigger than a souvenir card, maybe 1/2 inch-1 inch/ 1 1/2 cm-2 cm bigger (picture 1). Also cut out a piece of light weight iron on interfacing/stabilizer, just the size I want the label to be (picture 2). Lay the interfacing/stabilizer centered on the fabric and iron it on (picture 3). Fold over the left over fabric and iron it around the stabilizer, one side at the time starting with the long sides, to form folded squares that I will cut out in the corners (picture 4).