Firstly, here's the new loco. The cheapest available was a DRS liveried one. To remove the body shell, slide your fingernails alongside the edge of it, just under the doors, and release the four clips.
Naked Class 37! There are three pairs of pickup wires (red/black), each soldered onto the capacitor that sits across the two motor terminals.
These six wires need to be un-soldered, and the capacitor removed.
Once this is done, the six wires should be floating free, and the two solder terminals of the motor disconnected.
Now, you'll need a way to solder together the three red wires to the red lead of your decoder, and likewise, the three black leads to the black lead of the decoder. I'd suggest some veroboard, available from stores like Maplin. (I've only got "triboard", but it'll do!)
Solder the three red leads together on one strip, leaving a hole for the decoder wire. Solder the three black wires together onto another strip.
Now attach the red wire from the decoder to the "red" strip, and the black decoder wire to the "black strip". (I'm using a Lenz Silver decoder here). I've tied the five function wires out of the way for now as this model doesn't have lights.
Now solder the orange decoder wire to the left-hand terminal of the motor, and the grey decoder wire to the right hand terminal of the motor. This results in the motor end of the loco becoming the "No.1 End". Stick the decoder down on top of the loco weight using the sticky pad supplied with the decoder.
Test the loco on your decoder track, set the loco ID, and check it runs fine. Then replace the body shell, and away you go!