Evergreens rarely require pruning as they have a defined natural shape and are quite slow growing. When a drastic pruning from 12 feet down to 4 feet is required due to damage, competition with a roof structure or utility line, careful handling and thoughtful planning is called for. Your goal is to reduce the size of the shrub while not disfiguring it over the long term or causing shock. Aftercare will be critical to helping the evergreen survive and recover from such a harsh pruning.
Plan to reduce the size of your 12-foot evergreen shrub down to 4 feet in late winter or early spring before new growth unfurls to reduce the stress on the plant.
Shake or rustle well the top 8-foot portion of the evergreen intended for removal for a few minutes before pruning to alert any animals living in the shrub to evacuate.
Use your loppers to cut away the green branching and expose access to the main trunk just above the 4-foot mark. Tie a rope snugly around the trunk above this cut line and have a partner on the ground hold the other end of the rope to guide the 8-foot off-cut down to the ground as you make the cut.
Take the evergreen down in pieces when you are in a compact space or are not working with a partner. Cut the full 8 feet to be removed in 6- to 12-inch pieces at a time, allowing each smaller piece to fall to the ground.
Trim the top of the evergreen with your loppers or shears so that is has a symmetrical and pleasing top shape.
Clear away all of the cut debris and weeds from around the base of the now 4-foot evergreen so the soil is clear and exposed. Lay down a 2- to 3-inch thick blanket of aged manure and compost to feed the soil and water in well until the soil is drenched but there is no standing water.
Lay down another inch of shredded bark or leaf mold to act as a mulch to preserve moisture and keep weeds at bay. Water again in seven to 10 days and keep the soil evenly moist until the evergreen begins to show new green growth.