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A little more explanation would be in order... pictures are nice, but words help!
Hello James, let allow me to share my similar experience into Mr. Prasert's very useful topic. I can simply say we can use any kind of paper, but I prefer for a thin carton sheet (no matter whether they're new or ex-used).
Assuming that the base was for 1/35 scale, draw your desired pattern onto the sheet (commonly rounded, rectangle, or squared) by using an assistance from a 1/35 figure's foot to make sure that they're logically fit (not under-size, also not over-size).
Cut them one by one carefully. If you prepare them in a large amount, do not forget to collect them in a safe container. On your ready-to-used ground base, glued them one-by-one carefully using a nipper. Let them dry for a couple minutes. After that, your cobblestones are ready to be fashioned!
Sorry for not using images, but I hope this link can also be useful for additional references:
Build Log:"Komm und nimm es dir!"Diorama, just see directly to post #9. Since I also want to improve my techniques, can't wait also for Mr. Prasert's more representative illustration and explanation about making this cobblestones
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Thank you all
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Kind regards from Indonesia,
Garry