tutorial on my method of making flagstone bases. mainly documented this for my own future use, but figured I'd share here in case anyone else was ever interested. I should note this is by no means a quick or efficient method, and probably not one to follow for something like an army, but it's a way I enjoy doing it so i don't mind the time investment.
Step 1, cutting stone: gather up some cardboard, the kinda lumpy textured type you find in packaging inserts and egg cartons, best if you can get varied thicknesses. Cut the card into rectangles and squares, some small some large, whatever size you want the stones to be. For even more variation cut a 45 degree angle on some corners and a notch out of others to make it an L shape.
Step 2, assembly: glue the stones on in whatever pattern you like, try to keep a good mix of large and small pieces, as well as letting the various heights show. After the glue is dried seal the card, I use gloss varnish but something like mod podge would work as well, the point of the sealing is to ensure moisture from the future steps doesn't make the card go soft and lose its shape.
Step 3, texture paste: there's many products that fill this need, including just mixing sand and glue, but personally I use Vallejo earth texture. The goal is to end up with it in the crevices but not on the top of the stones, you could manually apply it to every single line, but that takes a type of precision and patience I don't have, instead, i just spread the texture across the entire base, then before it dries I wipe the excess off the tops with a paper towel, can also use a damp qtip for more controlled cleanup.
Step 4, undercoat: if you're smart this is where you prime the base, but I'm not smart so I almost never prime anything. For this step I instead use an off white craft paint, don't need solid coverage as the main goal is to get the texture in the cracks so washes will stain better later, but really, be smart and just prime in white instead.
Step 5, base coating: Get a bunch of earthy tones, here I have 8 (chocolate brown, grey brown, khaki, tan, off-white, light grey, 2 dark greys) then somewhat at random paint the stones with each color, with 8 paints I did 1-3 stones per color for 20mm and 3-5 stones per color for 40mm. the goal is to not have any area overwhelmed with a single color, so repaint stones as you go if the original choice doesn't fit anymore.
Step 6, washes: I used agrax earthshade and nuln oil. Apply the brown first, wiping off the tops of the stones, then once dried do the same with the black. The goal is to darken the crevices and lightly stain the stones (though the picture of this step only shows the agrax earthshade)
Step 7, drybrushing: Using an off-white I dry brushed the entire base, not to heavy but still ensuring the raised details were all picked out.
Step 8, wash again: Agrax earthshade once more, cover the entire thing, using a damp brush to soak up really heavy pooling on the tops, but let it sit for the most part. tinting everything slightly brown is the goal, in order to tie all the stone colors together.
Step 9, another wash: this time athonian Camoshade, applied to the crevices to set them apart from the browns of the stones. In the future I'll probably apply this more selectively rather than to every crack though.
Step 10, green: Sponge on some green paints, I used a dark green followed by a lighter touch of bright green. this is in prep for the plants, so do it mainly where you plan to place those. In some areas I went a little too heavy with the dark green, less is more in this step.
Step 11, vegetation: I use foam flock for moss, mainly placing it in the spaces between the green sponging. Grass tufts or static grass, applied in small patches around the base. Leaves, I use a leaf shaped punch on paper, and then stain them my desired color using a mix of washes. Though not shown in the picture if you're adding any extra bits to the base (in this case a crate and barrel) add them before the next step.
Step 12, pigments: I use chalk pastels, a selection of various browns and greens. I mainly apply the pigments to the moss and grass, to help blend them in more and add a bit of variation. I also apply a bit of grey around the barrel and crate to blend them into the base, and the same will be done for the mini once they're glued in place.
And it's done! There's minor things I'll tweak as I continue to use this style of base, but overall it gives the unkempt city street look I was aiming for. One thing i didn't mention before is the hole in each base, the minis these will be used for have a pin in their feet, so the holes are pre-drilled to match up where they'll be standing, and once glued down the pin will give extra strength on the connection. If you've read this ramble of a guide to the end, then thank you, and even if you don't follow it entirely I hope it could at least spark some ideas for your own projects!