All you need to do is look down the blowgun when you're ready to fire. You will see two blowguns because of the way you're eyes are positioned. Aim just in the middle of the two blowguns. At short distances your aim will always be correct but you will have to raise the blowgun up according to the distance you are aiming.
This is my first instructable and I will eventually follow up with an instructable on how to build a blowgun.