To take the chill out of a crawl space, it's usually best to insulate the exterior masonry walls rather than the underside of the floor above. That's especially true when heating ducts or pipes pass through the space. Unless there are excessive moisture problems in the crawl space, use unfaced fiberglass batts to insulate the walls; be sure to wear a long-sleeved shirt, gloves, eye protection, and a dust mask when handling the itchy stuff.
Cut lengths of insulation to extend from the mudsill (that's the 2-by board laid directly on top of the concrete wall) down the wall and 2 feet out onto the floor. Secure them with 2x2 cleats: Hold the cleat over the end of the insulation and drive nails through the 2x2 and into the edge of the mudsill.
Place a length of 2x4 or a few bricks on top of the insulation batts along the bottom of the wall to hold them in place. Also, cut pieces of insulation to fit between the joists and tightly press them against the rim joist.
