Boric acid works on insects both as a desiccant and as a poison. As a desiccant, it dries the insects' waxy layer so internal moisture leaks out. Termites eat wood, so injecting the boric acid into the wood kills them when they dine on the wood.
Boric acid is normally a powder. Termites eat wood from the inside out, so boric acid, to be injected into the wood, needs to be dissolved in propylene glycol (a non-toxic version of anti-freeze).
Purchase boric acid as a solution, dissolved in propylene glycol (e.g., Bora-Care). If you can't find this solution, you'll have to mix it yourself. Recommended ratios range from 1:1 to 8:1 (solvent:boric acid). Propylene glycol is sold in small containers at Home Depot and pool supply stores. Also, try marine supply stores, as propylene glycol is used for winterising boats.
Apply the solution on and near termite damage, with either a brush or a spray pump. Soak liberally. The solution needs to soak the wood thoroughly so that it penetrates into the wood, as termites eat wood from the inside out.
Bait the termites by mixing boric acid with sawdust and laying it near termite damage. Because this is bait, it should be sweetened to make it preferable to other wood nearby. Mix some honey and/or molasses in with the sawdust so the boric acid is not a repellent. This also helps the boric acid adhere to the termites.
Wood can be purchased pre-treated with boric acid as a preventive measure. Consider using diatomaceous earth (from the hardware store) or termite-eating nematodes (from your local nursery) if repeated application of the boric acid over three months does not succeed.