How do you stitch an open wound on the inside of your for arm?

Simple Treatment. Stitches (sutures is the medical term) are used to hold different layers of tissues together to allow for healing in a non-stressed manner. Depending on the depth, a doctor may use absorbable sutures on the deeper layers to minimize the tension and allow healing to progress. Skin sutures should never be tight as the skin is there to make us look pretty, not hold things together.
Closure. I would not do that yourself- open wounds need medical attention and a professional to stitch it closed (may or may not be indicated). See a physician to determine if closure is necessary or antibiotics is indicated.