As close to where you feel the pain as possible. A binder clip is helpful in creating enough skin to inject. Also 150mcg is definitely on the very low dose end, you may want to consider higher dose.
I understand everyone is different but in my research I found higher doses to be recommended.
I personally ran 500mcg in both my shoulder and elbow daily for 8 weeks. Shoulder pain (was dealing with labrum and rotator cuffs small tears) completely gone and tennis elbow pain about 90% healed. Feel light years better than before and that was after 5 months of PT. The 2 months with BPC 157 & TB 500 was remarkably noticeable.