> Question is exactly where to put them? I know there is a technique using a mirror. <
There are two issues. One is bass trapping, and as T said you'll put those panels straddling the corners. Use the four inch thick panels for that. You should probably put some of the two inch thick panels in corners too. Then you'll spread the remaining thinner panels around on the walls.
As for the mirror, that's not strictly needed unless the room has angled walls. For a plain rectangle you can easily calculate where to put them. You'll need either three or four panels altogether. One panel goes vertically on each side wall, halfway between you and the speakers front to rear. So they're not quite besides you, but a little forward of where you sit.
Then do the same on the ceiling. If your speakers are close together you can put one panel "wide ways" on the ceiling, again halfway front and back between you and the speakers. If your speakers are farther apart you'll use two panels on the ceiling, oriented the long way front to back. Below is a photo of my living room showing where I placed two panels on the ceiling, in the center of the photo at the top. You can also see stand-mounted panels on the sides, and some corner mounted panels too.