Use a cotton fabric, if that is what the quilt is made of. Labels can be just a plain rectangle, or something pieced, some are small works of art in their own right.
Many feel that all quilts should get a label, that way, gerations from now there is some history to go with the quilt.
At a minimum it needs your name, and the date the quilt was finished. If you want add laundering instructions. Dedicate it to someone. I just designed one for a step grandson, and put something like. For Noah, from Pam and Grandpa. 2009 Albion, Illinois.
If this is a quilt you will be putting into shows you may want to consider making a large labels so that you can list all of the shows it was entered in.
If this is a quilt that will be sent off to many shows, please add your address on the quilt, that way everyone knows where it belongs.
Make sure the edges are turned under and it is securely sewn to the back of the quilt. At many shows labels are required.