I've been asked this a couple times and thought I'd answer here. Thanks to "Tom Buckalew" on Youtube (https://goo.gl/a14uY2) for asking this recently. It sparked something in me to give a response, so here it goes.
It's real simple to get started. The beauty is you have an AR-15 and the upper can be removed from the lower and the bolt can be removed to provide a good visual through the barrel. Secondly, as a rule of thumb, affix the site forward about the distance of your magazine well. It's a good place to start. Don't place it on the hand guard however, because it has a tendency to shift when firing and won't hold zero as well as just mounted on the upper receiver section. Lock it down. As for sighting in, results will vary slightly but if you sight your .223 dead on at 50 yards it will be about .6 inch high at 100 and dead on at 150 and about 1.5 inches low at 200. So how do I do that do you ask? Simple.
Mount your upper minus the bolt and charging handle to a vise.
Look through the inside of the barrel at an objet 50 yards way.
Now look through the red dot and turn the turrets to line the dot up to as close to the identical picture you have when look in the barrel and be sure the red dot is center of the intended target.
Now it's not perfect but you will come close enough until you can get lead on paper to really dial it in.
I'll do a video about this soon to demonstrate. Stay tuned for more. Thanks guys! Thanks Tom!