Space out the pruning over three years since the shock to the tree has to be spread out. Never take off more than a third of the growth at one time or the tree might not recover. Once the tree is pruned back into a controllable size, you can prune it each year to keep it in shape and healthy.
You need to remove all dead wood immediately. Even if more than a third of the tree is dead, it will not further stress the apple tree if you remove the dead material. Cut back the dead branches until you find green under the bark. Old dead wood will only invite pests and disease to the tree as well as block sunlight from the healthy branches.
Cut out the water sprouts that grow straight up from the main branches. Often these straight branches will grow the year after your first heavy pruning. Don't be surprised if they grow quickly, reaching a height of 5 feet in one summer. They will rob the tree of energy and shade the branches below.
Season is important when pruning an old apple tree. Make your major pruning cuts only when the tree is dormant in the late winter. Minor pruning can continue in the spring and summer for a crowded branch or maybe a diseased section of growth.
