Step 1: The expensive parts
You can get a hitch and mount it yourself, or a Uhaul, Sears automotive department , or similar will sell you one and mount it for you.
The next part you will need is a "truck bed extender" I got mine from Harbor Freight.com (p# 39168-2VGA) for 30$, it now lists for $39.99.
The third part you will need for a minivan or car is a 2" square steel tube that fits into a 2" reciever hitch, I got mine as a left over from construction at work.
The last part you will need is another draw bar pin like the two that come with the bed extender. I got mine cheap at Home Depot, but harbor freight, jcwhitney, or any trailer hitch type place will have them.
This picture is from the Harbor Freight catalog, I used to haul my canoe upside down on the bed extender just like the boards are shown in this picture.