There are many ways to grow Potatoes, growing in a bag is one of them, you can use something like Miracle-Gro Gro Your Own gro-sac, which is easy to use and can accomodate 5 seed potatoes.
You need to plant five tubers at a depth of 20-25cm (8-10in), evenly spaced and then cover the tubers with a further 10cm (4in) layer of compost. Depending on what grow bag you are using, you may be able to place the potato bag in a bright, frost free position and watch them grow. Some bags require you to add more compost when the plants start to grow and shoots emerge above the surface, to cover them and then repeat as needed until the compost is about 5cm (2in) below the top of the bag. Keep the compost moist at all times, but don't saturate it as this might cause the tubers to rot.