I realize it's been over 3 weeks since you asked your question, and hopefully you have found answers in the meantime.

Basically, you can use any type of styrofoam stacked up as a form, but it should have the general shape you want to end up with. Mix up 1 part portland cement (not concrete) with 2-3 parts sand and enough water to get a thick paste that won't slide off the styrofoam. Apply this paste all over the top and sides of the styrofoam that will be above ground. Use a trowel to smooth some parts or put crevices where you want them. Then take a wire brush and use it to brush the cement if you want striations or tap it if you want a smoother stipple effect. Generally, stone in a watercourse has a smoother surface. The finished stone will be concrete gray, so plan on painting it with a masonry paint or better yet an epoxy paint. Use a rough shaped sponge to dab on pale grays, ochers and umbers. Try to mimic the colors in real rocks. Before you get started I urge you to watch YouTube videos on this subject. Even when someone is using a different method or material you can learn something.