I personal use a system where you use a temporary ring that attaches to your shooting glasses. You adjust that ring, which has been placed on the dominant side, until you can see the bead with opposite eye through the middle of that ring. That ring now gives you the place to put the translucent dot which blocks out the barrel from your dominant eye.
I posted a link above that gives you an idea. This is very simliar to the system I used. I think it comes with a dot that has the black ring to align the dot on the glasses.
Clean your gun and do not wash your hands. Put your glasses on, mount your gun and hold the gun with your right hand and move your index finger near your glasses so that it covers the end of the barrel. Lightly tap your glasses and put tape over the small oil mark left by your finger.