Roll green stuff 1mm thick long 10-20mm, 1.5mm on think side, take a piece of plasicard 0.5mm, water the green stuff, roll it over in lines every 0.2mm-1mm or more.
Alternatively instead of rolling it over in lines, get a small (0.2-0.3mm) tube and poke it to look like bumps/suction cups.
Finally, let it dry for a bit (a hour or two, depending no how much blue vs yellow you use) then apply before it hardens to much, just enough to keep shape.
OH, PS you can just buy something to do it for you from Greenstuff world, video https://www.youtube.com/watch?v=InIiFlWarrY
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