I would soak in a sink , towel press out some of the moisture , then put in the dryer in a delicate bag ( they are mesh bags , you can buy at most general stores like walmart).
Most of the shrinkage occurs in the dryer, so its important to do that step in pre-shrinking.
I preshrink lots of stuff( backings, battings, fabric ...even scraps ) when I want it to stay as flat as the day I sewed it.
When I want the crinkles look , I preshrink the fabric but not the batting, if both the batting and the fabric are shrinking you don't get as much crinkle.