Potatoes are just swollen stems, so seed potatoes are no different. Start by planting them very shallowly, just below the surface of the compost in the bag.
As they start to grow, slowly add more compost around the new stems, always leaving a shoot or two above the surface to photosynthesise.
As they get into more vigorous growth, keep adding compost until the plants reach the top of the bag.
The whole idea behind this sort of cultivation is that the covered stems produce lots and lots of potatoes, but they do need to be fed and watered well when in vigorous growth if you want a good crop.
At this time of year, the tubers need to be protected from frost, and they will only start producing sideshoots when it begins to warm up. Ideally, I'd 'chit' or start the tubers into growth in the normal way, putting them in a cool, frost-free place in trays, with any 'eyes' pointing upwards. At least you know when you plant them that you have vigorous tubers that are already growing.

Hello Birtie, I would hate to disagree with you but I was always taught to plant potatoes six inches deep and when the first leaves show re-cover with more soil/compost and keep totally covering them until you reach the top of the bag.

Don't think it really makes much difference, Ian, as the new potatoes are formed on the growing stems, not near the original tuber. In fact, last year I tried the method of "planting" the tubers on the surface of the soil and covering with little mounds of rotted straw. As they grew, kept mounding them up with more straw.
Worked very well, with a good crop that was extremly easy to harvest as you just pulled the straw away, but there was a problem of some tubers going green where I hadn't covered them enough. I think the 'earthing up' stage is probably the most important bit as the tubers that form must be completely excluded from the light.