this is pretty strait forward on how to make these, i find it the easiest out of all the methods i tried. first off find yourself and old baby nipple(s) like the one below made out of silicone rubber.

Lay it in some egg create upside down, like so.

make up your cement batch and pour it in.

and in a day or two later your plug will be ready, it pops out very easy, the pic below shows what it looks like.

there are different size nipples out there so look around and pick out the ones that work best for you. the nipples are reuseable many times, thats what makes this so easy, and there is no mess like with the sand form method, and no waste like the paper method.