Keyhole Garden - How to make an African style raised bed

I ran across this video people have been talking about on the permaculture site. While Montana isn't Africa (thankfully -- I don't need all the heat, bugs, and wildlife that they have in Africa. We have enough dangerous predators, thank you), we do have some problems with dry conditions. It will pretty much stop raining around July 1st and we won't see much in precipitation until September or October. The problem around here is that it's hard to create a decent garden with this soil. A raised, keyhole garden sounds perfect.