you can write one with a permenent pen- good ones that are recommended for this are Micron Pigma pens- they are acid free, archival quality- do not bleed, fade or smear- are made for writting that will last.
you can print labels with your computer- you can embroider them-
there are books of iron on labels you iron to fabric then fill in- there are premade labels you can buy in quilt shops (thimbleberries puts out some nice ones)
after you make your label you simply hand stitch it to the back of your quilt---anywhere you want it.
there are even people here on the board who for a fee will make a label for you.
quite often i print off a picture from my computer onto prepared fabric then i write on the picture...every label is as individual as every quilt....it's just important to remember the label
my avatar was actually used for a label on a quilt i made for my sister.