Oh! and please don't forget TA Only BEFORE you descend
You wanna do WHAT??!
Come off it, below FL200 the decent is no longer critical, and below FL150 it's a bonus. Supplemental O2 makes this even better. Adjusting VS to avoid other bits of tin is ESSENTIAL.
Discussing the 7700 option last few times I've done this in the sim with my operator, we decided it was purely for bonus points. Rightly or wrongly.
Once established in the decent, safe, and on masks, you need to speak to ATC and get out your PAN/MAYDAY. If they subsequently want 7700, they will ask for it.
The thing about emergency descents is, they don't always ruin your day. A few that occurred in my company resulted in the flight continuing to destination, albeit at FL100. So it's a matter of balancing reason, damage, fuel burn, comfort, pax state, crew state, maintenance at dest etc etc...
As always in this game, it's safety vs commercial all the time. By if you don't have to land immediately, and 2 hours at FL100 is an option...... Just sayin