It takes two people to put the sternum strap back on. You will need 3 tools: a pair of needle-nose pliers (or just regular pliers if you do not have needle-nose pliers), a medium sized flat blade screwdriver, and a hair blow dryer. 

First, take the pliers and slightly flatten the very top of the cloth-covered rod to which the "lobster claw" clamp attaches. Then warm up the "lobster claw" with the blow dryer to make it more flexible/pliable. You do not have to get the clamp super hot, just warm it enough to make it pliable. 

Then put the flat blade screwdriver into the opening of the lobster claw and twist it enough to slightly open or widen the jaws. Have your helper hold the cloth-covered rod with the shoulder strap and slightly twist the rod away from the shoulder strap and hold it and the shoulder strap while you put the clamp onto the top-most part of the rod while opening the jaws with the screwdriver. It is not easy, and it may take you a couple of attempts. 

Kelty has a video on YouTube; here is a link: