On the very rare occasions that I have felt the need to prewash a precut, I did exactly what Prism suggested. Keeps fraying to a minimum and the act of getting them wet then drying with the iron will shrink them a bit with minimal damage to the fabric. Just be gentle, gentle, gentle with the fabric. And instead of wringing them out, press them gently between a towel after washing in the sink to get the majority of the water out. The iron takes care of the rest and smooths them out. Good luck!
