There are two kinds of cobblestones: round and square. The round is easy if you're doing 1/35 or 1/32 scales. Using white glue, glue dry split peas in a regular pattern (e.g., rows), leaving a small space in between. Paint appropriate color. The square ones are actually stone cubes and they are regularly shaped. I create those cobbles by pressing into soft auto putty a tool that I have made out of square brass tubing (about 3 mm square). Again, I follow regular patterns. If you want to get fancy, you can make semi-circular patterns. Many square cobbles leave no space in-between the cubes. For those, I just draw a grid with a hobby knife on nearly dry auto putty. The little nicks the knife makes give a realistic look.There are also cobblestones that are irregular in size. You'll find those in old medieval streets, castle courtyards,etc. I press a wood tool in soft auto putty making irregular patterns and shapes as well as surfaces. I paint these varying colors, as the stones would be like that.