Write a script for the movie. Estimate about a minute per page as a rule of thumb, though the actual time may be a bit longer or shorter depending on the amount of action compared with the amount of dialogue. If you have ideas for the look of a scene as you're writing it, sketch it out in a notebook. Follow up the script with a storyboard sequence capturing the visual aspects of each scene, coupled with notes reminding you what scene the picture is assigned to and what specific action is happening.
Record dialogue using a digital microphone and audio recording software. Separate each piece of dialogue into a separate track, and give it a file name under an organizational code you will understand. Work with your voice-over actors before the recording session to make sure they get the feel of the characters and the proper voice. Edit the recorded audio clips for speed and pitch if needed.
Open your animation program concurrently with the audio program, and use your drawing tablet to draw the frames for your cartoon. Take care of the key frames first: the defining positions in animation that show the most distinct changes in movement. Use the audio track as a guide for creating precise mouth movements at the right times. Take a progressive approach, drawing the most obvious key frames and filling in new key frames between each one until you have approximated the rough movements of the movie. Use the fill-frame function of the program to create the remaining frames, if your program has that function. Otherwise, continue the progressive framing method to create a smooth, finished product.
Import the animation into a video editing program and lay it onto the program's time line. Import the dialogue tracks and place them on the time line in sync with the mouth movements of the animation. Export the project into a finished video file.