I don't prewash. At all. But for the last round robin, they wanted prewashed stuff. I put the fabric in a big mixing bowl filled with water as hot as it would come from the tap and two small drops of dish detergent. Swished & soaked for about three minutes, then rinsed with cold water and ironed dry. Since then, I've decided NOT to join any more pre-wash swaps! LOL. So, my recommendation is to NOT prewash.