It's smaller than I thought -- I don't know why I thought the box would be bigger than this, but I put a pen in so you can see the size, sitting on my keyboard:
Inside is an instruction book:
The first step is you go to the site and register your serial number. Then you (ahem) "Gather DNA." This means you spit into a test tube until it reaches a certain line. It says you can't eat, drink, smoke or chew gum for 30 minutes before you "gather." You probably shouldn't be swapping spit by kissing, but that's a fairly safe bet for me.
Then there's a vial of "stabilizing liquid" that you pour on top of your spit and shake it. Put it in a sealed plastic bag and off it goes in the mailer: