Think of pushing up the tail end of the deltoid. Do not proceed with your arms fully extended at 180 degrees; instead, keep them slightly at about 160 to 170 degrees. After completing 20 reps as one set (0 sets) with just your body weight, progress by adding weight. It's not about locking the joints; relax the joints a bit and think of your arms as being locked in a slightly bent position. Do not focus on the weight in your hands. Imagine carving out the tail end of the deltoid. If you proceed in the posture described above, the trapezius will naturally lower, so there won't be any strain.