The main ingredient is foam. Since I have been buying Flames of War models, I have a fair bit of the thin gray foam squares that comes in those boxes and that is perfect for the markers.
To make the explosion, roll up the foam along the straight edge. When rolled, add a drop of super glue and glue down the end of the roll. I was rally surprised at how well this material works with the super glue! Do be careful tho, the material is porous and you can easily glue your finger.
To make one "explosion" I use two strips of the foam. So, grab another strip and add it to the one you just glued. After gluing that strip on, glue the blast to the washer and you're done.