Wash the scarf in a washing machine set on the hot cycle. Check often during the cycle, as shrinking happens quickly and you don't want the scarf to tangle. To prevent scarf from getting tangled, use nylon thread to sew scarf in half, lengthwise, before you wash it. Remove thread when scarf is almost done "felting" in the machine.
If the washing machine didn't completely felt the scarf, finish it off by donning rubber gloves and placing the scarf first in hot water and then cold water. This also helps felt the edges. Lay the scarf flat on towels to dry.
To embellish the scarf, spread it on top of a Styrofoam work surface. Arrange pieces of wool yarn in whatever pattern you desire on the felted scarf. Using a felting needle, attach yarn by poking through the layers of felt.
Knitted wool scarf, twice as long as you want the finished piece