Begin by removing the shell and filing the metal tabs found in the front of the frame like so:
Remove the section highlighted in yellow on the left.
Next separate the trucks, gearing and motor from the frame.
Next select and perform the type of motor isolation you will use. For this installation I clipped the motor tabs and taped the frame but either will work. For more information on the ways to isolate the motor check these two links
Motor Isolation by Tape Motor Isolation by Cutting the Tabs
Now place the front of the decoder in the engine and approximate the length of orange and gray to allow for them to slip down the middle and connect with the motor.
Now remove the motor and decoder, clip, strip and solder the wires to the motor.
Next reinsert the motor (taking care not to pinch the orange and grey wire) and reinstall the trucks and gearing before putting the two halves of the frame back together.
For this installation the back end of the decoder is a bit to long, file or sand the back end of the decoder until it looks like this:
Be careful not to knock off parts or mess up any traces.
Now install the back half of the decoder, approximate the lengths of the black, blue and yellow wires, and solder them in place.
Finally replace the shell and enjoy your "U-Boat"!