I have got some GW Stirland Mud, some grass, some rocks and some grass tufts (Citadel Middenland Tufts and Army Painter Woodland Tufts
Sounds like you where going for rocky soil with grass tufts.
I would start with a bit of glue and then drop in few rocks and then douse the remaining glue with sand.
it's good to have a nice wide container for your sand and other flock. after it's all dry, remove it from the sand and brush off the excess.
at this point I like to prime my models just to seal in all the basing, but if you already painted your models, don't worry about it.
Ok now, paint your sand a dark soil color, and your stones a dark stone color if you want grey for your stones mix in a tiny bit of blue.
wait for this to dry, and don't miss any spots, you may have to go back an paint spots you missed.
no go back a dry brush on a lighter soil color with the side of your brush, be sure to leave some of the dark soil in the recesses. and do a lighter stone color as well, probably only do the tops this time.
you can use an old brush with no paint on it and maybe a tiny bit damp to help blend in the lighter stone. ok one last time do a highlight for your soil and your stone, at this point you can add a layer of green to represent moss or very short grass, I recommend a dark layer and a highlight.
Ok now all the painting is done, now you can use your grass tufts, just put some drops of super glue on you base where you want grass, might be a good idea to cover up some bits that may not have turned out so great or maybe they just look a bit dull. also you might want some tweezers to help you place your grass. (the same basic principle works if you are using flock as well)
Ok last step, go back and paint the sides of your base and cover up all the mess you made.
trust me it will look way better, even if you just paint it black again, other popular colors are brown or green.
