Standard foil sprayed with Pam will work. Just make sure you coat the whole thing. For applying texture to the steps, you don't don't need more than a single layer of foil. I recommend doing it in sections. Anything bigger than about 12" x 16" gets too hard to control when applying it to the surface.
I used high-strength mortar mix without any additional Portland cement. The only additive was the coloring agents. Only a tiny amount of color is needed to properly tint the mortar to a good stone-like color. Don't mix it too dry. You don't want it runny, like gravy, either. More of a pudding consistency. Smear it onto the Pam'd foil, about 1/4" to 1/2" thick. Wet the steps a little before applying the texture to insure good adhesion. Lift the foil by two corners, and slap it up against the step. Then press the foil down, smooshing the mortar into the surface a bit.
Remove the foil after the mortar has reached the "green" stage of the cure -- solid, but not fully hardened. You can wait until it's completely hard but it might be a little more difficult to pull the foil out of the creases. I sometimes used a small wire brush to blend the seams between foil-textured sections.
On my steps, I used the foil method on the front of the steps, and just spread a thin layer of tinted mortar over the top of the steps with my hand. This produces a surface which was rough enough to provide good traction even when wet. However, you could use the foil method on the tops too, it's up to you.
BTW, Bob Treat's article on using concrete to simulate stone on his layout is what gave me the idea for texturing my steps and tunnel interiors. I've seen his layout in person and the simulated stone mountains look great. I would have used his method on my layout if I couldn't get real stone. One thing I might have done differently than him, is add a small amount of color to the concrete mix. I think this would prevent chipped or worn areas from standing out.
