Carbon dioxide (CO2) is a byproduct of oxygen and sugar. So the less sugar you have in your body, the less CO2 that can be produced from exhaling. I'm just being overly detailed and cautious to make sure I do this right the first time. You probably don't have to do this. But it can only help the process.
You exhale, completely empty your lungs, then hold your breath while putting the mask on. The reason for this is so your first inhale after holding your breath for however long it takes you to put the mask on, will be 99.9999% nitrogen. There's no oxygen in your lungs (and you have little sugar in your body), so even if a little of the exhaled air stays in the mask, there is no CO2 in it because there was little to no oxygen (again, CO2 is a byproduct of sugar and oxygen). CO2 is what causes the suffocation sensation, which you don't want.
Doing it this way means you will likely lose consciousness after two or three deep breaths, or 10-15 seconds. You'll likely feel "high" after the first breath too since there is no oxygen. I'm still working on my lungs via the gym because I need to be able to hold my breath comfortably for around 30 seconds to have some leeway and error margin when I'm putting the mask on. Right now I can hold my breath for 28 seconds. So more treadmill and elliptical trainer!
When I started this CTB journey, I could only hold my breath for 15 seconds, so I'm making good progress. Once I can hold my breath for 30 seconds, I will be ready.
And yes, all the same with an exit bag.