Open the lid of your piano and use the prop to keep it up.
Adjust a microphone stand so the collar points toward the strings of your grand piano. It's imperative to get a clear, loud signal from the piano.
Fit a microphone to the microphone stand collar. Connect the female end of an XLR cable to the base of the microphone. Connect the other end of that cable to the "XLR Input" jack on the front of an audio interface.
Connect the audio interface to your computer with either a USB cable or FireWire cable, depending on the make and model of the interface. The audio interface converts audio into data, which can then be read by your computer's digital audio workstation.
Double-click the desktop icon to launch your preferred digital audio workstation, such as Cubase or Logic.
Open a MIDI channel and an audio channel from the file menu. Name the audio channel "Piano In" and the MIDI channel "MIDI Out."
Click on "Piano In" to highlight the channel. This assigns subsequent edits to this channel rather than the session as a whole.
Open the "Plug Ins" menu, typically located at the top of the interface.
Select your preferred MIDI trigger plug-in. This type of plugin lets you assign an audio source as the MIDI trigger. So whenever that particular audio source reaches the digital audio workstation, it tells the MIDI interface to create a note. Each workstation program has different plug-ins for this purpose. For example, Digital Performer's MIDI trigger is called "MOTU."
Adjust the "Threshold" settings in the plug-in interface. The threshold determines the volume at which the audio triggers the MIDI. If you want all notes to trigger MIDI sounds, set it low. If you want only loud notes to do it, set it high.
Click "Send To" on the "Piano In" channel. Select "MIDI Out" from the drop-down menu. This routes the audio from "Piano In" to the "MIDI Out," effectively rendering the audio signal as a carrier-wave for MIDI.
Click on "MIDI Out" to highlight it.
Open the "Instruments" menu. Select your preferred sound from the drop-down list -- for example, "Spooky Synth" or "Orchestral Strings."
Click the "R" icon on each channel to enable them for recording. When you hit "R," the audio from the microphone will record to the audio channel and the selected instrument sound will record to the MIDI channel. The note lengths and velocity will be the same on each.