1, You screw the plate on the right into the front beam with the opening facing up
2, You put a (say) nylon plug into the inboard end of the Spi pole
3, You drill a hole in the center of the plug the same dia as the fitting above (in the middle)
4, You place the plate to the left over the fitting in the middle and then insert this into the spi pole.
5, You then push the square end of the fitting in the middle (now pointing out of the end of the spi pole) into the gap between the front beam and the fitting in 1 above
6, you then slide the left side fitting over the other two.

I'll try and find a picture