Use a good microphone to begin creating your robot voice to make the best signal possible. Plug the microphone into the audio input jack of your synthesizer. Plug your headphones in to get a sound out of the synthesizer. Plug them directly into the synthesizer or connect the synthesizer to a stereo system using a mixing board.
Set your volume to a good level and check the signal coming in and out of your system. Your voice input should not be going into the red zone in the level of your mixer as this means you are getting a distorted signal. Adjust the levels starting with the input level on your synthesizer, then your mixer and finally your stereo system to get an optimal signal.
Select the vocoder program on your synthesizer to add a carrier to your voice. Keep in mind that your voice is the modulator and the synthesizer sound is the carrier. They are combined to generate an unnatural sounding voice that you can craft to your preference. Use the keyboard to activate the altered voice. Press down on a key or several keys as you are speaking into the microphone. You will hear a signal note or chords, depending on the program you select and number of keys you depress.
Experiment with different synthesizer sounds and listen to the variations created. Control the overall blend by adjusting the amount of voice versus the amount of synthesizer in the mix. Reduce your actual voice input level to get a more drastic effect.
Adjust the carrier signal by using the parameters in the synthesizer. Change overall pitch, add distortion or other effects, change the EQ settings and keep listening to the changes in the overall sound. Try pronouncing different words and decide how clear you want your voice to be. Avoid using too many effects if you want your voice to be understood.
Settle on a sound that works with your voice and sounds like the type of robotic voice you want. Play melodies by using the keyboard as you vocalize into the microphone. Save your settings and practice your lines to prepare for recording.