1) To put on the new string, simply slide one of the bullets into the nut, then place the other bullet into the appropriate saddle and push down on the back of the saddle until you hear, or feel, a snap.
2) This sound is the spring-loaded sliding latch snapping into place. If you don't hear or feel the snap, push the sliding latch toward the saddle until it catches. Making sure the sliding latch is all the way forward to its locked position is important because if it isn't, two things will happen:
a) The replaced string will be held sharp, and
b) The safety catch will not be engaged, so the string could suddenly and unexpectedly be released and fly across the room.
3) Now that the new string has been properly installed you should seat it. To seat the new string, press down firmly on the string just in front of the saddle and just in front of the nut.
4) Then do a couple of stretching pulls on the string.
5) The string is now seated and no more stretching is necessary; just play. In fact, if you stretch the string too vigorously, you will damage the string. Just play the string in for a bit and re-adjust your tuning with the fine-tuning screw as needed.
If the string you're replacing was tuned to your desired pitch, the new string you install should be able to be tuned to that pitch with the fine tuner screw. If the fine tuners do not have enough range to tune the new string to the previous pitch you will need to adjust the range tuner setscrew as described below. See: "Setting the fine tuner range on the SpeedLoader Tremolo."