Stewie Griffin from Family guy has a very distinct higher pitch voice with a posh Brittish accent and disdain in his manner of speech. He has some breathy tones in his voice once in a while and goes from higher pitch voice to a lower pitch voice and vice versa in an instance as he’s transforming from a fake nice to a ruthlessly mean and honest persona.
Use a higher pitch voice
Stewie is a baby character and has a higher-pitched voice appropriate to one. To mimic his voice raise your pitch quite high. You can practice it by reading some scripts in a higher pitch voice. You should feel the voice resonating more towards the upper part of your face mask. It should feel light and airy.
However, once in a while Stewie will go from his higher-pitched voice to a lower one or vice versa, that’s especially common when he’s going back and forth from mean to ‘nice’ Stewie. In these instances, the change in pitch is very sudden.
You can practice it in this script of Stewie whereas he’s being mean to Gavin and instantly switches to a nice Stewie when speaking to Brian:
Gavin, that's a stupid name and nobody likes you at school. You're going to be ugly when you grow up and everything in your house is cheap and it smells in here. Oh, hey Brian! Bye!
Punchy
Stewie speaks in a punchy way. He often emphasizes the end of a word or a sentence, especially when he’s angry or mean. It’s not always apparent, but when he’s being mean it’s very noticeable as he’s trying to put more weight onto each of the words to make it more hurtful.
Posh British accent
Stewie speaks with a posh British accent, especially when he’s nonchalant and cruel at the same time. You can watch some tutorials on how to do a posh British accent to nail the Stewie voice impression.
Annoyed and disdain
Stewie often acts as if everyone around him is below himself and has a disdain for other babies and people in general. When doing his impression insert some annoyance and disdain in your voice to match the manner in which he speaks.
Stick your neck out
For Stewie’s voice, it’s a good idea to stick your neck out. It naturally adds some tension, raises the pitch of your voice and thins it out.
Practice
You will most likely sound absurd at the beginning but it will eventually get much better. Keep practicing the Stewie voice. Watch plenty of clips of Stewie and try to repeat his lines in the same manner. That’s probably the best way to learn to do Stewie’s vocal impression. Also, record yourself to hear how you sound. By recording yourself you can track the improvements you’ve made and notice some smaller details that otherwise, you might miss.