Sorry, there's really no "easily" about it. You might be better off with
Flash or something for this, but I don't think it will be exactly easy in
Flash, either.
Motion paths will help you in PPT, but they don't make the shapes morph into
other shapes -- in other words, motion paths aren't going to be able to make
a straight-ish rope image into a curved rope image. It might be better to
just use images of the different stages of the knot-tying.