Just be sure to either order the extra velcro piece to "blind" the one piece of velcro, or get a piece from a store to do so - otherwise, the velcro between the strap and the body will abrade your seatbelt - I doubt it matters, but I got tired of the wear on my old seatbelt.
You can either undo the strap, and fasten it around the shoulder strap, so it stays attached, or you can feed the seatbelt buckle between the strap and the body - I much prefer to feed the buckle through, as I prefer that position, and feel its much more secure.
I really, really like those things for carry, and have given them to various family members.
It might not be subtle to a cop, but I don't think anyone else will notice, and you really can't get any more "ready" even if you open carried a pistol while driving.