container spuds?

How do I grow potatoes in a barrel Ie do I earth them up as they grow or plant them as i fill it up?

 

Also how big does the barrel have to be.

 

Fill the barrel half way with compost, one of the large half barrels will do.
put about 5 pot's in evenly spaced, cover with compost, water.
when the top growth is about 2" high, cover with more compost, repeat till the barrel is filled to within a couple of inches from the top of the half barrel, leave to grow on normally, watering as required.