It's similar to the pronunciation of "hello," but with a slightly longer "o" sound.
Here's how you can break it down:
* hə: This is a very short "uh" sound, like the "uh" in "uh-oh."
* ˈlō: This is the stressed syllable, with a long "o" sound, similar to the "o" in "go."
So, "halloa" is pronounced with a short "uh" sound followed by a long "o" sound.