usually you remove the new growth, dip in hormone rooting powder and plant. don't try and take any of the tuber with it. as the cutting grows it will produce its own tubers for next year.

Seaburngirl's advice is spot on, if you're wanting to increase your stock of dahlias - if you just want the same ones to grow again this year, pot them up now, you don't have to take anything off the tuber, just make sure its got a big enough pot until its ready to go outdoors in the ground. If its remaining in the same pot for the summer too, the pot needs to be large enough to accommodate a tuber that will be bigger than it is now - it will increase in size again this year.

Thanks to everyone