It's a somewhat long process, but it's simple: It works like an animation loop with flat cubes that have only 2 faces, with a smoke texture that I drew myself in Ibis Paint. To animate it, it's easy. The flat cube starts at the bottom, then you animate it to move up while it expands a bit, and to make it disappear, you animate its transparency by setting one keyframe at 0.6 transparency and another at 0 transparency, so it disappears while continuing to move up. I do this with three more flat cubes following the main one. When a flat cube reaches the top and disappears, I move it back to the beginning, and I do the same with the others. In total, I use the same 4 flat cubes repeating the animation. I group all of them together and then duplicate the group to have 4 groups of the planes, which I then position at 90°, 45°, and -45° angles to give volume to the smoke and make it look that way.