James,
Please see this post for some photos of it. Sorry for the extra clutter on the page but I'm sure you'll get the picture:
Make sure that the boom is solidly on the boomkin and lash it in place (to hold it down) then take your main halyard and attack it to the boom at the point you'll be attaching the chain hoist. Tension the main halyard so that extra support is provided for the boom at the point the weight will be put on it.
To attach the hoist put several wraps around the boom at the main halyard point. Attack the chain hoist to the wraps and at that point you're pretty much all set. In the photos above the wraps around the boom were pretty crappy because they were just wraps with a bowline connecting both ends. However, since those photos were taken I've done it again and I used a Portuguese bowline to make the boom wraps. A far superior design as it distributes the weight.
Hope this helps and good luck!
Tate