Here's an article from Poets.org on writing haibun. Alison is going to post some more links.
More than the Birds, Bees, and Trees: A Closer Look at Writing Haibun, by Aimee Nezhukumatathil
Ah, I found the longer haibun that's been haunting me, that I keep mentioning--will probably make my way through the others on this list of what one editor looks for, at any rate.
A Fistful of Frost