The pin (part#108) goes in front of the threaded ring on your lower reciever, the spring (part#109) goes in the hole first. The pin retains the buffer inside the buffer tube .
The pin goes with the wider end down on top of the spring. After you load the recoil spring into the tube, you then place the buffer into the tube while depressing he nib down on part #108 andreleleasing it once the buffer is in and under spring pressure. The small spring,part#109 will push up part #108 and the nib on this part holds the buffer in place.