Some tips.
There's a book SM 101. Get that and have a read.
Get a collar for your sub. Lovehoney sells them.
Collar on = playtime starts.
Collar off = playtime finishes.
It means you have it defined what is being done as play and what isn't. When you are in the role and when you aren't.
As post above use "amber" for heading out of comfort zone, "red" for "stop right now." Otherwise we are Green and ok.
Going beyond that if subby isn't saying amber but you were spanking hard and still nothing (for instance), you can say "On a scale of one to ten, one being unhappy, ten being fine, where are you right now?" It may be that having pushed children out of her mimsy she can take pain and is fine.
If mouth is blocked, for instance you have tied up subby, and are now forcing your tiggy deep down her throat then she isn't going to be able to say anything. So put something in her hand and keep an eye on it. Perhaps it could be a ball and ball dropped from the hand means stop. Or get a squeaky toy put it in her hand and the toy being squeaked means stop.
Get a pair of these:
https://www.lovehoney.co.uk/product.cfm?p=36753
Or similar. Used by paramedics and good for cutting someone out of whatever, if you suddenly find you need to release someone quick in an emergency.
Blindfolds are used to eliminate sight which means other senses are enhanced.
Bondage is restraining limbs and removing control.
Note some things can enhance. Do you want to tie up subby and have the subby see the toys being laid out? Then blindfold? Or do it from the start and have it as a surprise?
Avoid moving someone once blindfolded or in a spreader bar. Tie hands behind someone and they can't save themself if they fall forward. So I avoid moving someone once play has started.