To be able to felt it you need to remove ALL the dirt and grease.....so.....
you will need.....
bucket/bowl/sink
hot water min 60 degrees c
soap flakes
washing soda
rubber gloves if sensitive skin
Disolve good shake of soap flakes and soda in water and gently stir and squeeze....DO NOT agitate too much or will felt in the water

Tip out or squeeze and remove and then repeat....water should be not as grubby as first wash!
Rince several times in water that is not under 30/40 degrees C as plunging into cold water will shock it too much.
Finally put through mangle or put in spin dryer.......if you only have a modern washing machine you can use the spin cycle ONLY if it does not tumble it a lot during the cycle as that will agitate it too much.
Finally dry it in sun/ over radiator/ near wood burner / in front of rayburn or what ever you can use to dry it quickly but without touching the heat source.
Then its ready for carding!