This syllable, has absolutely no counterpart in English.
One book says it's a "reduced schwa." That helps a lot, doesn't it?! They say to pronounce it like "uh-ee" said together quickly.
One web reference had the following formula, which I really like:
Step three is the hardest part. When you say "oo" your lips just want to spring back to a round O shape out of habit. You may actually have to hold your lips in place with your thumb and index finger the first few times you try this.