Mine were not hugely successful last year. This year I've done it differently.
I put about 6 inches of soil in the bottom, then the spuds then about another 6 inches. When the green shoot came through I kept adding more soil until the bag was full. Mine are now about to flower so fingers crossed I got it right.
I got some potato bags from the garden centre (3) Followed the instructions and had 12 potato plants in about two weeks. We even shoved one straight into the garden soil, just to see, and that also sprouted at the same time...
(we don't have much else going on in our garden though)