Pop Song Subsitution by Quantazelle
Pick a pop song you enjoy that you can copy easily (or find a multi-track MIDI file). Try to make it as close-sounding to the original as possible--use drum samples or synth versions of the original instruments. Try to mix it as close to how the original sounds. Keep every instrument on its own track.
Once you're happy with this facsimilie, create a duplicate track under each instrument, but replace the recognizable instrument with a new sound or synth that's innovative and completely unexpected, but works in the same "space" as the original track. I'd recommend using synths instead of samples, because when you start mixing the tracks you'll start to notice muddy parts or sound cancelling, and by having a high level of control over the sound on a granular level. Keep mixing and tweaking until you've created your particular cover of this pop song that has your imprint.
This will be fun! Remember, however, especially in the US that this is still a cover song and if you decide to commercially release it you will need to hit up Harry Fox or other agencies to make sure royalties get distributed to the original artist if you decide to sell the final product.
I hope that this project will show you, on a hands-on basis, how pop songs are created, and also *from what* they are created, with the hope that you will recognize popular structure and keep pushing forward into more awesomeness (sound-and-structure-wise) to create amazing songs with your particular take on them (and not just as pop songs--I just chose that parameter as a focusing agent).
Have fun, and keep pushing the envelope as well as our sonic expectations.