Proper Tefillin Placement
The Bayis shel yad must rest on the center of the width of the biceps muscle of the weaker arm leaning slightly toward the heart.
According to the Shulchan Aruch and the Rama, no part of the bayis shel yad may rest any higher than the midpoint of the humerus bone.
Some say that the lowest edge of the biceps muscle is determined when the arm is outstretched and relaxed. Others say that the lowest edge of the biceps muscle is determined when the arm is bent at approximately 90°. Consult a rabbi which opinion to follow.
Verify that the bayis is not positioned too low by locating the base of the muscle with one's finger and making sure that the front edge of the base is above it. If one can place the pinky, ring, and middle fingers between the cleft of the arm and the front edge of the base while the arm is outstretched, then it is most likely in the proper position according to the opinion that the base of the muscle is determined when the arm is outstretched and relaxed. If one can place the index and middle finger between the cleft of the arm and front edge of the base while the arm is bent at approximately 90° then it is most likely in the proper position according to the opinion that the base of the muscle is determined when the arm is bent at approximately 90°.
The entire bayis shel rosh must rest above the original hairline, which is determined by the hair's roots. It's suggested to allow a finger's width of leeway. One whose hairline has receded should continue to place the bayis shel rosh above his original hairline, even though hair no longer grows there.
Feel along the hairline with the length of the index finger or with the tips of his fingers to verify that the bayis is resting above it.
Draw the index finger and pinky or the index finger and thumb from the sides of the base down to the eyes to verify that the bayis is centered.
The back edge of the bayis can reach until the top of the head, which is equivalent to the end of the soft spot on a baby's head.
Identify this point by running the tip of a finger straight up from the front of the ear.
The majority of the knot must rest on the center of the base of the skull. The remainder of the knot may rest just below it in the indentation on the nape.
Verify that the majority of the knot is resting on the base of the skull by pressing on the upper part of the knot. If one feels the small, round, protruding bone when pressing, then the knot is in position. If by pressing on the lower part of the knot one feels that it's in the indentation of the neck, then that verifies that it's centered.
Source: Rabbi Yerachmiel Askotzy, STAM