This installation is for N Scale Kato SD40-2 uses the TCS K1D4 Decoder and was performed by TCS
First remove the shell by grasping the locomotive on either side, squeezing gently (be sure your fingers are not holding the guard rails) and gently wiggling it back and forth until it slips up and off the chassis.
Next, remove the lighting board by pulling it towards the back of the locomotive and lifting it up and out.
Now wrap a piece of kapton tape around the frame (all the way from the engineer side to the fireman side) above the motor tabs. The image below shows the completion of the process.
Reverse the process you used to remove the lighting board to insert the K1D4. First place it into the notches in the frame, then slide it forward. Make sure the motor clips on the decoder are fully covering the motor pickups on the locomotive. The fireman's side typically slides right over, but the engineer's side will require a bit more care. The following two images show the motor clip covering the motor tab and the K1D4 installed.
You can now replace the shell of your locomotive and you are ready to run!