Parts:

4 2 hole x 4 hole plates

6 8" x 1" bolts

6 1" nuts

Slide the bolts through two of the plates, the frame tubing, the remaining plates, and fasten the nuts.

Parts of the plates should stick out past the tubes as shown.

Leave them a little loose.

The pictures show a 7th nut welded on. You'll install that later so don't worry about it just yet.