I'm sure it's fairly simple...I used a PSX pad and converter on my PC b4 getting a wheel.
At what point are you having trouble? DO you know where all the Control Options are in LFS? Or is LFS not recognising it at all?
GO into Control Panel - Game Controllers, and make sure Windows can see your pad.
From there, go into LFS and Select Options - Controls. Select AXIS / FF option, and move your pads sticks; you should see bars on the bottom right of the screen moving to show you what your pad is sending to the PC. So - move your pad left and right, see which bar moves, and then click on Steering, then select the axis bar that was moving. Hay Presstoe.
Buttons work much the same way - Click on Buttons 1 or Buttons 2, click on which control you want (eg Horn) and then press the controller button you want to use.
Give it a whirl, see what happens.
Hope that made sense.