It is nice to have one of the fancy dubbing brush tools that you get now a days, but if you don't want to spend big bucks on it then there is a simple and cheap way to do it. This is how I currently make mine and it works well enough for me.
All you need is a blackboard pin, some wire, your chosen fibers (in this case SYN Fiber) and a sturdy dubbing twister.
First stick your blackboard pin into a piece of wood or as I did into my table top. Twist the end of your wire around the pin and roll the wire out flat on top of the table. You can make your wire brush as long as you like.
Cut your fibers in the length that you want and start placing you fibers on top of the wire. In this case I have placed the end of the fibers close to the edge of the wire, but you can place it in the middle or any way you want it depending on how you want your brush to look.
Carry on doing this until you have reached the length that you want. You can add different colours as you go along. Try to keep the fibers fairly spares and make sure that you spread them out over the wire to avoid and clumps when you spin the brush.
Once you have placed all your fibers on the wire take the end and hook it over your dubbing twister. Fold the wire back over the fibers so that the fibers are sandwiched between the wire. Twist the wire around your blackboard pin again to secure it. At this point keep tension on the wire by slightly pulling on the dubbing twister.
While keeping tension on the wire start to slowly twist the wire by spinning the dubbing twister. As the wire start to twist around itself lift the dubbing brush up so that it can spin freely in the air.
Keep on spinning the brush
Occasionally you would need to use a dubbing needle to loosen any trapped fibers as you go along. Keep on spinning the brush until you feel that the fibers are secured enough and wont be able to come loose.
At this point you can use a barbers hair comb to comb out the hair and
to loosen any more trapped fibers. Use a wire cutter to cut the ends of
the wire and you are done.
To use the brush simply fold the fibers to one side and tie it onto your hooks shank and twist it around stroking the fibers to the back as you go along.
The fly below was tied using the dubbing brush I just made.