Subcutaneous injections are given in the fatty loose tissue layer under the skin. There are very few reasons to give a subcutaneous injection to horses. In most cases, equine injectable medications are given deep in the muscle, or in the vein (by a vet).
Generally, horses have very little loose skin in which to give an SQ injection compared to other domestic species. That said, occasionally allergy shots and other medications are given to horses SQ.
Use the same landmarks for injection in the neck as was described in the IM injection detail. Ensure the area is clean. You may choose to swab it first with an alcohol swab.
Rather than directing a 1.5" needle perpendicular to the skin, use a shorter needle and enter the skin at a shallow angle, alongside elevated skin resulting from the skin pinch. As with IM injection, draw back before depressing the plunger to be sure you have not entered a vessel.