There was a how-to for these in the recent World War I themed Wargames Illustrated. Basically take round cocktail sticks/toothpicks, break them at various lengths and glue them to card. Once dry, push model railroad clump foilage down on to the cocktail sticks. Spray black and then overspray with a light grey: Instant explosions. Looks also like the
OP add some red to the bottom of the marker too.
QED.
I haven't made any of the big blasts, but I am using the same technique to make smoke/destroyed counters for
FoW.
Skriker