You need 2 nuts, 1 bolt and 1 washer that fit the thread of the rivnut. Put a clearance drill through 1 nut so that it has no thread and slides up and down the bolt.
Push the rivnut into the hole, wind the threaded nut and washer onto the bolt, then slide the drilled nut onto the bolt under them, then screw the bolt into the rivnut. It should go in far enough that the thread of the bolt is fully engaged with the thread of the rivnut. Tighten the threaded nut / washer down onto the drilled nut and wind it up tight, so that the bolt tries to pull the rivnut out of the hole, but it can't because the drilled nut is holding it in. The rivnut will then deform inside the hollow section as designed.
Loosen the threaded nut and test to see if the rivnut is tight - if not, wind it up a bit more.
Piece of ****.