I started with some simple oranges, then added yellow inside. The power and force generates from the middle and goes out. It's important to keep that in mind. Smoke, debris, fire, whatever, is all going to have a spot of origin.
I start from the middle and blend outward. I then ass white and do the same thing again.
At this point, I begin to overlap. The explosion is not a perfect bubble of fire. Fire and sparks have a mind of their own, so I tried to be spontaneous with some of my choices.
I start to make smoke with 4 shades of gray, painting dark to light. I randomly make spots and splatters of debris. Again, this explosion has a mind of it's own and layering the different elements of the blowout will help show force and power.
I start to build up more of the smoke in the foreground and expanding outward. The explosion goes in every direction so I try to keep depth in mind, there are things happening behind what we are seeing. I use the orange and yellow to blend into the smoke nearest the fire. This is to represent the light from the explosion.
I finish by breaking up some of the smoke. Smoke might appear dense but it's not solid. Boom! more debris firing outward to show motion in the detonation.