Start off by laying down a basic shape using soft edged brushes set to like 75% opacity or less, and for the thinner smoke at the edge or end of the smoke trail, add a bit of low opacity blue smoke, as tyre smoke is slightly blue.
Then thicken up the smoke in the centre, again using soft edged brushes and a lower opacity. Also use a smaller sized soft edge brush around the edge of the cloud, to make it more defined in places.
Then add some detail by combining the soft brushes and the sorta splodgy spatter random ones (
), with a whiter colour. Dust over with very low opacity soft brushes to slightly loose the harsh edge.
This all assumes you have Photoshop.. dunno what the whole brush situation is like in Gimp
. There is no real definite way to do smoke, and you can't just follow a set of instructions to do it