"Prewashing" batting, Jingle, isn't actually "washing."
Hobbs suggests putting it into a large capacity washer, fill with lukewarm or cool water, turn OFF the machine and let it sit about 15 minutes. DO NOT LET AGITATE, or then you likely will have a mess! Drain and spin out the water; again do not agitate.
The batting can then be "dried" in a dryer set at the "air only" setting to help remove wrinkles; then fluff it over the sofa (or a line if you're picky or lucky to have one) and go to bed over night. In the morning it'll be perfect for layering/basting/quilting.
With Hobbs battings I do this not so much for shrinkage, but because they are rolled and packaged so tightly that the wrinkles make them hard to layer for me.
Jan in VA