There exist basically two recipes for homemade grenadine on the web:
1. Add equal parts pomegranate juice and sugar and shake with cold/room temp.
2. Add equal parts pomegranate juice and sugar and boil 20 minutes until reduced.
I think both of these have issues. The first method may keep the "bright" flavor of the grenadine, but you will have to refrigerate the final product to keep it from spoiling. Oh right--another thing--adding a dash of alcohol does nothing as a preservative. You need at least 10% alcohol in order for it to do anything useful preservative-wise, and then you'd be making pomegranate liqueur, not grenadine.
The second method yields a "cooked" taste, and sometimes a brownish color. Based on my recent post about simple syrup, the long cooking with the pomegranate acid will indeed result in a long shelf-life product, but I bet the flavor suffers.
So I decided to make mine by adding juice and sugar 1:2, and then heating until clear--only 5 mins or so. This results in a thicker syrup great for those layered drinks (and partial invert, so it will stay good for a long time unrefrigerated), and yet doesn't seem to compromise the flavor or color. And the syrup is tasty! I need to get some vanilla ice cream immediately to pour this stuff on.