Well, you have got Jesters template for the spitfire.
But you want to make a desert scheme and the template only offers the temperate scheme.
So, now you need to draw the whole scheme yourself? No!
The base is pretty simple.
Select the grey base layer and fill it with the color you need. (Tutorial on working with layers
Next you choose the color for the undersurfaces and you select the layer that needs this new color
And when you have done the same for the camouflage patterns you suddenly notice that the sprayed edges are gone and you have all jagged edges.
This is easy to prevent.
Before you fill the layer with the color you want Lock the transparency of the layer by clicking the little button on the layer window. Note that you need to do that for every layer that you are filling with a new color.
Now the result will give just the sprayed edges as the original template had. And within a minute you have got your desert scheme!