The Tail is the second most important part of a fursuit, I never did finish furring this one but I'll go as far as I can with you on this.
I wanted something non traditional so I used vaporion as a reference with a this base and feathery finish. Draw up a basic shape and then add a few extra layers for a nice stable base. Then glue it all together, it helps to do the layers one at a time and pin them together as they dry. The layers don't have to be perfect (as you can see) because you will trim them down to get a nice smooth shape depending on your design.
Be sure that it fits properly so it can be nice and snug against your lower back (around the base of your spine)
Trim it nice and smooth (there will be foam fluff everywhere)
Create the pattern by taping or draping and cut the fur. Just like with the mask stich and glue the fur to get a nice fluffy tail than give it a trim if you want.