After washing them with plenty of running water, turn them inside out so that they are thoroughly cleaned on the inside, too. How? I have learned a method, that of the "knitting needle", but I know that there are others around. Use a knitting needle to push the edge of an intestine towards its interior and carry on pushing untill all the length of the intestine is passed through to the other side. This way, the intestine is turned inside out and you can carry on with washing it. If the intestines come from a suckling lamb this process can be "compromised" a bit.