Let's suppose you want to assign the S4 switch (only the S4, nothing else) to fire your guns and the rest with the A2A Accu-Sim Spitfire. This S4 switch is mapped as #4 in the Cougar viewer app.

1) first make sure FSX is not running ;
2) make also sure the S4 switch is no longer assigned to a specific FSX action in your Foxy .tmj file. If it's still assigned it can't work as a firing control in CSWeapon for FSX
3) make a backup of your current CSWeapon.xml file in case things go wrong...who knows ??
4) open your CSWeapon.xml file located in the WoP3_Spitfire folder (Mark Fletcher's mod applied, thanks very much to him)
5) edit this .xml file in the controls info section starting with :

<!-- controls section -->
       <Action name="fire" control="joystick:0:button:3" />
       <!-- fire button -->
       <Action name="masterSw" control="Ctrl+6" />
       <!-- weapon master switch -->
       <Action name="nextWeaponType" control="Ctrl+7" />
       <!-- weapon type selector switch -->
       <Action name="nextWeapon" control="Ctrl+Shift+7" />
       <!-- weapon subtype selector switch -->
       <Action name="nextWeaponMode" control="Ctrl+Shift+8" />
       <!-- weapon type fire mode selector switch -->
       <Action name="groundSw" control="Ctrl+8" />
       <!-- radar AIR-SURFACE mode selector switch -->
       <Action name="hudModeSw" control="Ctrl+9" />
       <!-- HUD mode selector switch -->
       <Action name="rangeSw" control="Ctrl+0" />
       <!-- Radar range selector switch -->
       <Action name="unlock" control="Ctrl+U" />
       <!-- drop a target  lock -->
       <Action name="fovInc" control="Ctrl+Shift+9" />
       <!-- HUD FOV increase  (2D only, AUTO in VC) -->
       <Action name="fovDec" control="Ctrl+Shift+0" />
       <!-- HUD FOV decrease  (2D only, AUTO in VC) -->
6) don't forget to save the new .xml file
7) have a try with the Spit on the ground or airborne : the S4 switch should work as expected as the only firing control.If you want more firing controls it's up to you : edit the .xml file by adding the corresponding lines to get them, as indicated in the CSWeapon SDK manual

Hope this helps some Thrustmaster Hotas Cougar owners