The hardest part of making this figure is finding the Commtech Stormtrooper. Once, you've done that, dig up an old POTF2 Sandtrooper for parts.
You'll want to cut the pauldron off the POTF2 Sandtrooper by cutting on the left side toward the neck, where it's thinnest. If you cut all the way to the neck, you will be able to pull the pauldron off from the right side.
Fit the pauldron to the Commtech Stormtrooper's neck, widening the pauldron's neck hole as necessary. Once it fits right, you can slip it around his neck without gluing it.
Next, take the backpack and cut off the pegs that hold it on the back. You'll have to cut down the part that lies along the back, since the Commtech Stormtrooper's back is a different shape. Because of all this cutting, it's more practical to just glue the backpack on when it fits right.
If you want, you can dirty him up by drybrushing with some brown and grey. I decided not to do that, because I figure there's got to be at least one Sandtrooper who keeps his armor clean.