Choose an 8X10 frame that has a fairly deep inset at the inner edge. This one cost $8.00 at Michael's. The edge is not too visible so I put the knife in there to try to show that this one is about 3/8" deep. That will give a nice depth for the water putty. Tape the edge to keep them clean. This frame had a fairly stiff back board so I glued that in place with some caulk. This also seals the edge to keep them dry in the next steps. This kind dries harder than silicon and hold the backer in place really well. Fill any holes made from the removal of the angled stand.
Since it is essentially cardboard I spread a very thin layer of the caulk to seal it so the water putty doesn't soak into the board and turn it into mush. Make sure to gets down into the where the frame and the backer board meet. again to keep a good seal to keep the backer dry.
Mix the water putty to the consistency of a thick milkshake. Stir is well be not to aggressively to keep the amount of air bubbles down. Pour this into the frame filling it all the way to the top edge. Tap the sides to get the air bubble to come up and to help level the surface. If need be take a straight edge and screed the top level.
Now it has to sit for a while and harden. You don't want it completely cured or you will understand the name of the product. This rested a little more than an hour. It is hard but not completely set and has a damp feel. It is just right when you do the next step and the surface spall's a little as you go. (That's when the surface of masonry flakes off)
Before the individual stones are scribed the surface of the stones needs to be added. I used a rock to make the impressions all over the surface. If the back board is not even with the back of the frame this will flex at this step and the putty will likely crack. I put a book that just fit in the space underneath for support so I could really crap up the surface.
With a straight edge, scribe all the courses first. The pattern I used is the most common for brick work called a running bond and is fairly easy to do. One even easier is called "Jack on Jack" which has the "mortar lines" in both directions running the length of the work. This is most commonly seen in something like bathroom tile. Each tile meets 3 others at any corner. Using a kind of a motion that feels like your hand is not steady as you scribe will help to give the uneven edge that is desirable. In the left photo oyu can see where some spots have the surface chipping off in little chunks. This can be controlled by how hard you scribe and if oyu kind of push to the side a bit as you scribe. If it's not there don't worry for now. Use a stiff brush clean off the loose stuff from the surface. You can make a lot of damage or none of you like.
Now scribe the individual stones. I marked off a bass wood strip with the width of each course and filled in every other space to use as a guide for cutting every other row. Scribe the row skipping one at a time them move the wood down and shift over one space. Scribe, moved down and shift back one space. Makes it real easy to keep things right. You can also "cut" any on the full size stones in a course into 2, sometimes 3, pieces. When these where put down there where odd sized stones that had to be used. The last thing to do is eyeball the whole thing and, using the scriber, chip up any edges that look to clean and straight.
A pretty easy process to make some nice looking cobblestone pavement. You will need a picture frame, Durham's Rock Hard Water Putty, some scribes, a bass wood strip and a sturdy straight edge. I would imagine this can be done with Plaster of Paris as well. I like the water putty because as its name says, this stiff is rock hard when fully cured.
The painting steps are easier than you might think it looks. The 1st coat is overall Tamiya medium gray. 2nd is a very mottled spray of Tamiya dark gray over most of the surface. Leave some medium gray as is. Time to vary the colors of the stones. I found a photo online of some pavement that I liked to use as a guide. I started with the Vallejo flat earth mixed with water to a very thin consistency which was applied with a flat brush. The trick is to keep it thin so it just tints the stone. You don't want it too opaque. This thinned flat earth was then tinted on the palette with each of the other colors, saddle brown, black and red black to get the variation. Believe it or not one of the hardest parts was keeping it random. Let this dry in preparation for the next step.
Mix the raw sienna and raw umber at a 2:1 ratio. To make enough to cover this 8X10 piece it was about a 2' line of the raw sienna as squeezed from the tube and of course half that of the umber. Mix this well with some Japan Dryer to shorten drying time. With a stiff brush apply a thin coat to the entire surface then go back and work the color into all the nooks and crannies. Let that dry for about an hour.
Fold a paper towel into a pad and wipe the surface taking off as much paint as oyu can. Don't be too firm wth the pressue so you don't wear away the acrylica colors underneath. There will still be a lot of paint down in all the recesses. Take a clean stuff brush and go back and forth over the spaces between the stones. This will clean some of the excess of of the grove and spread it over the tops. Wipe the tops off again. Keep going until the groves and depressions are dark but not all gooey with paint. Make a clean pad of paper towel folding it so it tight so it is hard. Barely dampen it with white spirits and very lightly wipe the surface to the stones. Side aside to dry for a while. After a dry brush with white to hight the edges, a coat of flat varnish finishes it off.