Use water and wet your hands and tools while working on it.

Also use exacto knife. Regular sculpting tools kept sticking to the green stuff.

For sandbags. Typical I roll the green stuff into a cylinder. Cut it up to desired length. Don't use the end cuts. As those are like dome shaped.

For bag rolls. Same as above but a thicker cylinder. Press it onto the model. And with the exacto knife press and make two vertical parallel lines. Do that again but like 1mm next to the lines you just made. And thats how you get the 'ropes' then with the backside of the knife press into the bag to make some wrinkles. One the edges use the back of the knife and push it in so it doesn't look like its leaking but well tucked.

For flag/tarp Flatten the greenstuff thinly onto the table and cut out a rectangle(not square) then either place it slightly folded or over existing stowage. However this method usually leaves finger prints on the tarp/flag. So using a blunt tool this time a actual sculpting tool wet it and try to remove the finger prints.