When Daniel saw the large, smooth "sheets" of cotton that came in the mail, he was pretty sure I ordered the wrong thing. That's because prefolds need "prepped." There's a few steps to take to make them absorbent and diaper-ready.
Wash prefolds on hot.You can add a small amount of detergent, but even that's not really necessary. I added a little in the first cycle, then not again for the others.
Dry prefolds on high heat.Use dryer balls instead of dryer sheets. After a couple cycles, I got tired of this and didn't let them fully dry in between. Although the high heat in the dry is what helps them quilt up and become absorbent.
Repeat until prefolds are quilted.Keep the wash-on-hot/dry-on-high-heat cycle going until the diapers shrink and quilt. It takes 6-8 cycles to reach max absorbency. They can generally be used in as little as 3-4 cycles if needed, but might be more prone to leaks at that point.
The photo above shows a brand new prefold on the far left, then a prefold after 1 wash, a prefold after a few washes, and a fully quilted and prepped prefold after 8 washes on the far right.