That stringing aid should not be difficult, you will figure it out with time..
Your string will stretch, on its own slowly or quickly if you shoot with it. Keep your eye on where it sits, once you find a place that makes you happy youwill have to take one end of the string off and turn it once or twice to shorten it back up again, this will be particularly important if you are setting up an Excal scope.
Congrats on the new bow by the way!!
You will probably get about 15-20 shots off the bow before the string starts to settle down and not stretch so much, so quick, get those ones out of the way before you do any serious tuning to the bow. It will still stretch after that but not as quickly.
Have fun!