Pick a location on the sloped area to plant your apple tree that is easily accessible, not too steep, and has good drainage. Apple trees can be planted in early spring or late fall. If your area does not receive a heavy frost, you can plant throughout the winter.
Dig a hole at least twice the diameter of the root ball of the apple tree sapling. When digging into a slope, it is necessary to dig the back of the hole deeper than the front so that the tree sits evenly in the hole.
Put some of the looser soil you dug out, or potting soil or compost you have, back into the hole to help the tree spread its roots and sit straight.
Set the tree in the hole and backfill the hole with dirt. On a slope, the front of the hole will need more dirt than the back. If necessary, use a few large rocks or a couple of pieces of firewood in front of the tree to help hold in the dirt while the tree is young. The stones will act as a retaining wall and keep the dirt from running down the slope, eroding the tree.
Mulch your tree well with straw, wood chips or compost, and water it deeply. Avoid adding any fertilizer after planting as this can burn the roots.