In October or November my dad ordered me a kit from Marshall's for growing potatoes in bags. His intention was that I would have home-grown potatoes for Christmas. But the kit only arrived a week or so ago, so today Steph and I planted them up, with some help from Eleanor and Rebecca.
You put about 8" of compost in the bottom of the bag then put three chitted spuds on top and cover with about another 4" of compost. Keep it watered, and as the shoots appear you cover them with more compost until the bag is full. I've had friends who used this method and they were amazed at the size of the crop. They told me the bag is just jam packed full of potatoes by the end.
The kit came with three bags and fifteen Swift seed potatoes. The instructions said to put three spuds in each bag, so we jabbed some holes in the bottom of a half-empty compost bag and planted that up as well. There's nothing magic about the bags that Marshall's sent me - you can grow spuds in any sack or bag or bucket of roughly the right size. The only "magic" is that receiving a kit through the post gave me a kick up the behind to actually do it. Thanks, Dad.