Oooh...I LOVE plaiting tails!
I found it easier to master than the mane.
You basically start of with 3 bits at the top and start the plait as you would normally, then you keep adding small bits in from each side as you work your way down the dock.
It's hard to explain! I've got a step-by-step picture guide from an old copy of Your Horse somewhere at home...I can scan that and send it to you if that'd help???