Not a leather worker BUT soak the sheath in water, wrap your knife tightly in a zip lock bag or other waterproof protector, insert the knife in the sheath and let air dry..while still damp, BONE IN by using an old toothbrush handle to press the leather into the knife contour. Leave in until completely dry...carefully remove your knife and be sure its dry, lube.
A real cowboy or other leather worker will come along with a better way however this will work..I think.