[mb6606]

I set two cheap door mirrors side by side against a wall.
I grab any club for this example a 5 iron. Setup in front of the mirrors so you can see the ball and right shoulder (down the line view).

Set a broomstick/dowel (broom stick is longer) in the frame of the mirrors. The broom stick or dowel is set at the angle of the sweetspot of my club up through my desired shoulder plane. You may have to re-adjust several times to get it right.

Swing the club back (RFP naturally) and down - you could also use a double ended laser, double flashlight or dowels. Watch your hands/shaft ride up and down the broomstick. Even if your hands are slightly above/below the broomstick no matter as long as they remain parallel to the broomstick (on one plane).
I like to do the dowel drills as well in the mirror.
"LOOK, LOOK, LOOK"!!!