It is not a basic question. Anway on this model the DCC decoder is in the tender. So you read the maintenance manual that comes with the loco and it will tell you how to get the tender body off. I did mine but I can't remember if it is one screw or two. Normally you unscrew the screws (you will need a small philips/posidrive head screwdriver) and pull the tender top either forwards or backwards (again in the instructions). Once the tender lid is off at the back of the tender you will see a rectangular socket with 8 pins. Into this there will be an 8 pin header, bit like in your PC, if you have ever opened it. Remove this header, and put the decoder header in its place. The decoder should have something to indicate pin 1, as will the socket, so make sure pin 1 goes to pin 1.If it is a Hornby decoder you will need to insulate the decoder by placing it in a bit of heat shrink, or a plastic sleeve of some kind, if you use some other manufacturer it should come ready sleeved. Now just put the tender back together and you are done. It usually takes longer to get the tender lid off without damaging it, than fitting the decoder.
I forgot to mention, you need an 8 pin decoder.