Find a pine sapling that is fairly straight and free of knots and bumps. It should be approximately 2 inches wide. Trim this sapling with the knife to no longer than the length of two of your arms. Any longer and the bow will be difficult to fire from the back of a horse. Strip the bark off the sapling with your hands and allow to dry overnight.
Decide which side of your sapling will be the back or the belly of the bow. Draw a straight line with a pencil down the back of the bow. Secure the bow in the vice with this line facing up.
Use the knife and file to give the bow some shape. A Mongolian horse bow should be wider in the middle and taper down towards the ends. The exact shape is up to you, but the bow should be comfortable to use. The widest part of the bow is the grip and should be in the center of the bow. When shaping the ends of the bow, be sure that they are at least 2 inches wide by 1/2 inch wide. Any thinner than 1/2 inch and the wood may not be strong enough to bear the weight and pressure of the bow.
Sand the bow completely smooth to prevent slivers. Use several leather straps to fasten the bow to a bench, belly-side up. The straps should hold the bow completely straight. Place the bench in the sun to dry for at least three days to gain the necessary flexibility.
Unstrap the bow and use the knife to carve a notch approximately 1 inch deep into each end of the bow. These are the nocks that will hold the bow string in place.
Varnish the entire bow with the varnish of your choice. Use a quality paintbrush and try to avoid brush strokes. Follow the manufacturer's directions to determine drying time.
Apply glue to the grip area and secure the leather grip in place. Trim to size if needed. Allow the glue to dry according to the manufacturer's directions.
Tie one end of the bow string to one of the nocks. Tighten the string and secure it to the second nock. Experiment a little to find the perfect tension. Once you've achieved a comfortable tension, the bow is ready to use.