You do not need to use the red rings on a modern mobo. They are insulators. Old 286 mobos needed them, but nothing now. Make sure you use the brass standoffs between the mobo and case, and only put a standoff where there is a screw-hole on the mobo so you don't short it.
When the heat sink is fully in, there should be an audible click, and you should be able to tell from the back side that it is secure. It is best to mount the heat sink before you screw in the mobo, to 1) avoid flexing and possibly cracking it, and 2) allow you to confirm that it is indeed secure.
You can set the mobo on a deck of cards or a roll of tape to lift it up off your table enough to mount the HSF.