Wookie Roar | Ask MetaFilter:
Chewie was actually a baby brown bear called "Pooh" whom, Burtt jokes, they didn't feed for a few days, then teased with bread soaked in milk. That sound was then overlaid with dogs and a stranded walrus.
So, first, get a baby brown bear, deny it sustenance for a while, and learn to imitate the frustrated growlings that result when you tease it with snacks.
Then, pay close attention to your dog's vocalizations.
Next, strand a walrus, and listen carefully. I'm sure we would all be very interested to hear your success in this endeavor. I stranded a walrus once, and, boy, do I have some stories!