We used four builders bags last year. We just put them on handy bits of level ground, and used a mixture of cheap compost and mushroom compost. We put a layer in the bottom, planted about 20 spuds in each, then added more compost as the plants grew.
We folded down the sides of the bags, since they were much too high, and kept them well watered.
One problem was that the compost in the bags bags became too warm in the sunshine, so you might want to protect them a little.
We got a decent crop of very nice looking potatoes. But the cost in compost was the equivalent of buying them wrapped in gold leaf at Harrods!
