Before you can create an animated explosion, you have to draw the spark. This can be anything from a grenade landing to a stick of dynamite burning down. Whatever the explosion or size, it must have some sort of ignition.
Animate this by drawing a series of pictures depicting the ignition preparing to explode. For instance, if the explosion comes from a stick of dynamite, you can show the fuse burning down slowly by drawing a shorter and shorter wick on each frame.
When animating the main explosion, it is best to draw the initial stages as bright and small. You can do this with small white or bright yellow cracks appearing across a grenade, for instance. After you have drawn a few frames showing the initial setup, you can create the main explosion. This should expand rapidly in just a few frames. Color-wise, it is a good idea to start the explosion bright yellow before adding red and orange flames to the outer area. You can even expand the explosion so that it becomes wider than the frame itself. During this time, it is a good idea to have any items such as furniture or debris quickly blown out of the way as the explosion overtakes them.
Every good explosion needs an aftermath. This allows the viewer to see the damage caused by the explosion and adds realism to the event. Smoke and falling debris can create the impression of vast damage. Animate this by showing charred black debris dropping down the frame rapidly while lighter ashes float. Make the ashes appear to float by changing their position much slower than that of the heavier debris. Rocking the ashes back and forth will also help this illusion.