Step 1 - Your hole should be twice as wide and only as deep as the rootball of the tree. Be sure to leave space so that the tip of the root ball is 1" to 2" above the ground level.
Step 2 - Remove the container. Plastic pots should be removed by gently laying it on its side and tapping at the pot until the plant slides out. I know it's common sense but I'll say it anyway -- Try to avoid damaging the roots while doing this.
Step 3 - Refill the hole with a blended mixture of 1/3 Peat Moss and 2/3 of your garden soil. Firm the back fill by tamping it gently. Build a watering basin around the perimeter high enough to hold 3" to 4" of water.
Step 4 - Your basin should be at least as wide as the hole that was dug. Immediately water the tree deeply by filling the basin with water once, letting it soak in, and filling it up a second time. Remove the basin after this initial planting / watering as heavy rains and excessive moisture can lead to root rot.
Provide winter protection for your young trees. A hefty layer of organic mulch such as straw, wood chips or bark should placed around the trees base. Do not pile the mulch up so that it covers the lower portion of the tree, it's the roots you are actually trying to insulate - piling up mulch on the bark / trunk can lead to other problems.
To protect the trunk a tree guard is a good idea, either a commercial one designed specifically for the purpose or even a home made one will suffice.
Heavy duty commercial paper wrap is one option of protecting young trees and saplings from winter weather. They are also useful in repelling some insects and mammalian varmints as well. Plastic guards such as ones rigged from PVC pipe are sometimes installed around the tree base . I don't advise hammering pvc into the ground as you'll probably do more damage to the roots than good, they also do not protect against pests. Spiral Plastic tree guards are helpful as well.
Plan for the long term, in all likelihood if left unmolested your apple tree will probably outlive you. If possible, Plant grass or a ground cover near to the tree base to prevent erosion. Grass cuttings will add organic nutrients to the soil . Do not let the grass grow too tall or it will compete with the tree for surface nutrients.