Welcome to Knitting Help.
I've always attached fabric to a knit blanket by hand but some of the advice here
for machine stitching applies to hand knits and may help.
If you decide to go with hand stitching
, this is a good stitch to use. It's a good idea to wash the fleece so that it has a chance to shrink or adjust before you attach it to the afghan.
Is this to be lining for the afghan?