If you look on the BD website they have a section on it. I think you have to have a recommendation from a judge and fill in a few forms, and then do training sessions etc. Not sure of the in's and out's but I think it may be a bit time-consuming!
Good luck - keep us posted as it's something I've been thinking of too!
I might offer myself as a writer and go from there. How annoying, you would think they would encourage people to train really, as I know in my area they are often short of them. People don't always have the contacts to be recommended!
I know someone who's hoping to be a BD judge. She started by volunteering to write at affiliated events and judging at unaffiliated events for the practice. Most unaffiliated shows are short of people who will judge. Try Croft Top, Arkenfield and Rosses