The carrier signal is what you'll actually hear out of the vocoder, so you'd use something like a synth or a guitar or something as the carrier. The modulator signal is what will give the vocal-like formants to the vocoder, so you'd most likely use your voice.
Maybe start with ReaVocode instead of TAL, because it'll make first-time setup a bit easier. Move on to others once you've got the concepts under control and can fiddle around a bit more effectively. Morphoder is OK, but not free. And it can't accept an external carrier, only its built-in carrier signals (or if it can, it's doing it in a non-standard way that I could never figure out). TAL and ReaVocode can both accept external carrier signals from whatever source you like, and they're free.
The main concept to understand is that you're sending from one track to another in order to feed the vocoder both the carrier and modulator signals that it needs in order to work. It's the same concept as sending a vocal track to a dedicated reverb track, for the most part. The complication is that you need to add additional channels to the vocoder track so it can receive the carrier signal.
In Reaper, you'll need to put the vocoder FX on a track and change the number of inputs on that track to 4 instead of the default 2. Click the routing button on the track and from the Track Channels drop-down, select 4.
This will be your modulator channel, so this is the track that you'd arm and record your voice through. Set the input accordingly.
Now create a 2nd track. This will be your carrier signal, so put some sort of synth lead or pad on this one for now. Click the routing button on this track and add a send to the other track that has your vocoder on it. Once the send is created, a new box will appear. At the lower left corner of that box, it says "Audio: 1/2 -> 1/2". That means that this track is sending its output channels 1&2 to the other track's input channels 1&2. Change it to say "Audio: 1/2 -> 3/4" because input 3/4 is where ReaVocode and TAL will expect the carrier signal to be on the vocoder track. And while you're here, uncheck the "Master send" checkbox in the upper left corner. You don't want to hear the carrier unless the modulator is actually doing something.
Now you can arm both tracks for recording (or just the vocoder track, if you've mouse-clicked your way through creating something in piano roll). If your carrier signal is holding down a sustained note or chord, you'll only hear it when you speak into the mic. And it'll be modulated by the sound of your voice.
