I usually take and make a pile of sand larger than the stone I am wanting to make. I wet it down and get it about the consistancy of wet beach sand where it gets firm, then I carve out a
basic shape to accomodate the concrete mix. After that I lightly sprinkle in some more damp sand and leave it lofty for an impression. I will set my hand in and press down evenly, even using my other hand to push my fingers in deeper. If it is not a good enough impression then sprinkle more sand in voids and impress again.
After this you can make a mix of Portland Cement and Sand 3-1 Mix and add water to the point it will be a thick pour ...about like waffle batter :) and carefully pour it in starting from center, but GET IT IN THERE! The sand mold will immediately draw the moisture out of the concrete mix and it will leave gaps in the face of stone so keep it moving.
Don't get too frustrated because it can take practice. Once you try this you will understand my challenges in pouring my large Sand-Cast Fireplace Mantel!