When I was starting out, I would keep my wires coming out of the back of the Heli and then attach the velcro tape to the other end. Slide it all the way until the velcro on the battery meets the velcro on the battery tray and then keep pushing it in another centimeter or so. You check for proper COG by picking the Heli up by the flybar.

If you STILL can't get it right, head on over to the E-Flite sub-forum here and ask your question there.