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Animators generally do the storyline and motion-video first. And if it looks good, the audio and music is taped afterwards. Don't do the voice first, then video after - it is waaay hard, which is why you find it hard. It's easier to sync your lips with the video, rather than the other way 'round.
A good example is Robin Williams at the start of "Mrs. Doubtfire", doing his job as a cartoon voice. See how he puts the voiceover on the cartoon? The cartoon was done first.
Hope this helps! (pay me back by checking out my profile)
stelth