When planting apple trees, choose a sunny, sheltered site. Space them at a distance that allows for their eventual width, and a bit extra (i.e. trees on M9 rootstocks can spread to about eight feet, and should be planted at least nine feet apart).
Do not position them where an apple tree has grown previously. You need to come outside the span of the old roots (about the same as the branches) to avoid "apple replant disease". Pathogens linger in the soil for some years after an apple tree has been removed, and these will prevent the new tree from thriving.
Dig a really big hole and place the tree in it, making sure that the graft (the knob on the stem) is well above ground. Spread out the roots and drive in a support stake. Refill with good soil bulked up with plenty of compost and a handful of bonemeal. Attach the tree to the stake with a tree-tie or a non-abrasive material such as a piece of rope fed through old hose. Water well and be mindful that it will need watering during dry spells for at least the first year.