Shower arm installation doesn't require the use of any special tools or technical plumbing knowledge. A water supply pipe runs inside the shower wall and an elbow attached to this pipe holds the arm in place. Both ends of a shower arm have threads on them. One end of the shower arm threads into the elbow inside the wall, and the shower head attaches to the other end. Proper installation ensures the connections don't leak.
Wrap the threads on the neck-end of the shower arm with plumber's tape. This will ensure a tight seal.
Slide the wrapped end of the shower arm into the hole in the shower wall until it meets the elbow in the wall. The elbow will have a female end and the shower arm will have the male end. Thread the two ends together by turning the shower arm clockwise until the threads catch. Don't force this process if the threads don't align right away. Forcing will cause cross-threading. Cross-threading strips the threads. If needed, shine a flashlight in through the hole to ensure proper alignment.
Wrap a hand towel around the shower arm and then grip it with a pair of adjustable pliers. Continue tightening the shower arm until the shower head end points down toward the shower stall floor. The fit should be snug, but not too tight.
Avoid over tightening or you could break the plumbing in the wall or ruin the seal created by the plumber's tape.