Sounds as if you could do with a glossary:
http://www.sweetwater.com/expert-center/glossary/t--DAW
But the principle is to record or import a musical piece in order to adjust, embellish and present it with the computer. If you're good on the keyboard you can simply play it in. Or you can download a MIDI piece which somebody else has made, which I would roughly liken to an electronic player-piano file. This can simply be altered or set to play through outboard MIDI equipment or software instruments.
Most sequencer programs have a 'piano roll' MIDI mode in which you can plot or amend your tunes before rendering or printing as notation.
Plotting the music from scratch, with mouse, can be a very disconnected and robotic affair. Though those Assembler guys know to have fun with less, so who am I to complain? There are plenty of Amiga trackers out there with countless MOD files in their pockets to prove it (MOD is similar in principle to MIDI), so you may want to Google around.
Though plotting or not, I would consider it imperative to own and connect a music keyboard to your computer using MIDI. The joystick port on the computer (if there is one) would be ideal. It's much easier to dab your work with the human touch where and if needed. (And given many compositions being exactly tailored to the old computers, much data would be lost in transition, meaning a lot of information being re-interpreted and played again for the new work.)
To take a look at a creative use of mouse-driven piano roll, see my page on Instant Music for the Amiga and C64:
http://www.dustybin.org.uk/IM.htm