Since: Apr 08, 2004
You'll want to import the instrumental piece to start with. Either import the file into a track, or drag the file onto the center area of the reaper screen. This should create a new track with the song inside it.
Sounds like you already have that part.
Now, you want to insert a new track (CTRL-T) to receive your new incoming signal (your singing).
Once you have a new blank track, you want to ARM that track for recording. Do this by clicking on the little circle at the far left of that track. The circle should turn red. You should un-arm the instrumental track, as you don't want to do anything to this track.
Once the second (voice) track is armed, you should mouse over the input meter, and click. There should be a hint message, saying "Set Recording Input Channels". This will bring up a window that lets you set from where reaper should listen for signal.
Often you would set this to be Input 1, or Input A, which means the first input on your sound device.
Now, if you want to hear your incoming signal, along with your instrument track, then click on the little speaker, below the ARM circle. This will tell reaper to send the incoming signal (voice) out the outputs, so you can hear it along with your instrument track. Sometimes this is helpful, sometimes not, depending on how your hardware is set up.
Before you press record, you should see the input meter jumping around when you speak into the mic.
Now press record, and sing away. The instrument track won't be touched, and your singing will land in the second track.
You can do this over and over (make new track, arm it, set it's input, and record) so you can add in different parts without having to be perfect all the way through.
When you play, all the tracks will play back at the same time.
Each can be muted, or solod, so you can hear exactly the one you want.
When you're done, and are happy with your tracking, you can mix each one to sound how you want it.
After mixing is done, you will want to render all the tracks down to a stereo wave file. Click on File -> Render to make this new stereo file. If you're going to burn it to CD, then render the project down as 16bit / 44.1khz or else the burning software won't burn it to CD.
Hope that helps. Shout out any other Qs, as I've been in reaper for a while now. I'm no expert, but I can do the basics =).