Apples should be pruned during the winter months while the plants are dormant. Immediately after planting, trim back the grafted main branches by a third, always making the cut just above an outward facing bud.
For the following 3 years, prune the tips of the main branches and sub branches by one third in winter. Aim to develop the main branches to form the permanent framework of your tree, with fruiting sub branches growing off of them. From the fourth year, some sub branches can be pruned out at the union where they join the main branches, to allow new sub branches to develop and take their place. The sub branches will produce the best quality fruit.
I know this sounds complicated but there are lots of useful diagrams online and you are essentially pruning to create a bush tree. Just bear in mind that when pruning family apple trees you will need to treat each of the grafted stems individually as the different varieties are likely to have different growth rates. Remember to remove any suckers that develop at the base of the tree each year as well.