First off, grab a hot glue gun, and secure them to the base and to one another. I formed mine up in rough squares. Don't worry if you cut them imprecisely and end up with different heights. You can either rationalise it thus: some are in depressions or on rises in the ground - or fix it in the following step. At this point I also glued a single 1/35 barrel to the base.
Second, get some tissues/newspaper/similar, PVA, water and a cheap brush. Mix up the PVA and water. I think I used a 50/50 mix (you can add some paint to this mix if you like or wait until later; you can see I did on one fuel dump and didn't on the other). Apply to the tissue, and place that over the fake barrels. Apply it more thickly over the shorter barrels now, if you want to equalise the heights. If you've used a single proper barrel model, don't cover it over. It's there to suggest to the eye that everything on the base is an actual barrel.
Third, once that's all dried, apply some sand to the base, secure it with PVA, and let it dry. Next get plenty of gauze bandage, dip it in a PVA/water mix, and apply it over the tissue layer. Tear or cut a few small bits of paper, and place them atop the gauze. Once that's dry, you can paint everything, and add what ground cover you desire. I knocked these up as I felt I didn't have much in the way of fuel dumps, so when I went to deposit them in the attic terrain store I naturally found masses of the things. Anyway, here's how they look.
