Tips for growing potatoes in bag
To maximise your yield, start your potato tuber off in half a seed tray with compost of your choice. Use pure compost, or a soil/compost mix. Adding mycrorrhizal fungi to your compost at this point may help. (this is a powder that encourages root growth and is available from all good garden centres).
Let this grow on a window shelf, or greenhouse, away from any frosts. When you have some good strong shoots and leaves you will find that the tray fills with a matt of roots. This is the time to transfer to your potato growing bag. Put in a bit of compost into the bottom of your bag and then add your potato. Add a little more compost to cover the roots and the tuber. Roll down the sides of the bag and water. As the potato grows, add more compost, and roll up the sides of the bag.
Alternatively put your tuber straight into the bag on top of a layer of compost, roll down the sides of the bag, and plant the tuber. As the potato grows you can add more compost and roll up the sides of the bag.
Potatoes are hungry. You will have to feed your plant well. A general fertilizer or tomato feed works well.
Do not over water your bag initially, as the plant is living mostly off the tuber. Once the last frosts have disappeared the bags can be kept outdoors, and water as required. The earlier you plant your potato, the better chance you have of getting a big yield. If your stems get too tall you may have to support them to stop them being damaged by the wind. Happy growing.