For these felt balls I used the wet method of felting.
Have the water on as hot as you can handle.
Take the piece of wool roving and a droplet of soap in you hand. Roll and roll some more.
Continue to roll, rinsing your felt ball occasionally.
When your felt ball has gotten as small as it can and your hands are red from the hot water turn your water to cold. This will help shock the wool and your hands. Do one last hot/ cold and continue on to your next felt ball.
Let the felt balls sit out over night or in the hot sun to dry.
String and hang your felt ball garland! I used a bright blue embroidery thread.
Any heavy duty twine would work too.
The felt balls pretty much stay where you place them on the string so there is no need to knot them in place.