Anyone who has ever had to try and install the DeLorean battery strap when it was all taken apart, or put together incorrectly will agree with me when I say this is one of the most confusing and ill-conceived parts of the car. I think I spent less time installing my new water pump than I did this strap! The hardest part is figuring out how to loop the strap through the clamp, it is pretty much impossible to describe in words, so I've included some pictures of how to do it properly. Keep in mind I have never seen a DeLorean with a "properly" installed strap, so I could be completely wrong, but it works for me.
Once you have looped your strap through the clamp as the picture shows, slip the hook on the other strap through the top hole on the clamp as per the picture. Now remove all of the slack from the lower strap by pulling on the loose end. Once it is fairly tight, but still with some slack, push the clamp handle down until it stays down on its own. You're done!
One of the caveats I've found is the size of your particular battery. The upper strap doesn't allow for much tolerance since it's not adjustable, so if you have a really odd sized/shaped battery you could run into problems. The problem being the bottom of the clamp will hit the floor of the car and not allow you to close it all the way. You might have to get creative here to get it right. I still had the plastic handle on my battery, so I ran the strap over that which took out just enough slack to allow the clamp to fully close. The battery is now held to the body very well and is impossible to jiggle around.
Strap Looping Configuration (The Hard Part)