Get a friend or two and lift the grate on top of the hopper.
The front end (with the slots) should be as shown, and the grate should be inside of the hopper on the other side (the side with the nuts).
Have your friend hold the grate in place for the next step, or use C-Clamps to clamp the grate to the supports.
Bolt the grate to the hopper supports (1/2″x 2.5″ bolt and lock nut). Bolt both loosely, and then tighten them.
You’ll need 2 washers on each side, as the holes are slots and a 1/2″ washer will be too small to prevent the washer from warping. Use a 3/4″ washer as the other one.
You may want to use a large nut in place of a washer on the hopper support side. (see photo)