tight as you can get it. take a rag and put over the prop and hold the prop while you tighten it.. as long as you dont put a pipe on the tool your using, your not really gonna break anything. just get it as tight as you can by hand. dont really know how to explain it any better

PS, dont forget the prop washer!!! i seen a few new guys that didnt have it on, thats kind of iffy.