I label all my quilts except baby quilts. I put "name of quilt" Made for _____ by his/her mother/grandmother/sisterinlaw/cousin etc my name, then the month and year and if it's quilted by somebody else I add at the bottom of the label "professionally quilted by soandso."
I don't label baby quilts because little kids have a tendency to pick at things and might pull it loose.
I use white muslin and cut it half an inch bigger all the way around than a piece of Wonder Under. After the WU is fused then I write my label with a fabric pen. I turn the edges under and fuse it to my quilt and then do an inivisble stitch around the edges.