Go to the hardware store and get pick up a 1/4" clevis pin with holes in it. slide pin in from right side of reciever.
Line up the first hole with hole in the reciever and place your spring and detent in.
Push the detent and spring down with a small punch and start to pull the clevis pin out a little to capture it.
Place your pivot pin against the clevis pin and start pushing it thru.