Torrid is pretty much straight forward in their method.
Measuring band size, or where your bras would end on the bottom coming around your chest. this should be fairly snug.
Then measuring bust, or where the outer most edge of your nipples and coming around your chest. If you have fat rolls like I do this measurement should be snug as well for proper sizing. If you are fairly lean then just pull it taunt.
Then you need to do some math:
I'll use my measurements as an example.
My base line, or the lower measurement where the bottom of my bra will ride is a snug 50"
I measure the bust line by coming around my chest from my nipples just taunt enough for the tape not too fall it's 60", but I have lots of fat rolls and side boob. I will pull it snug because of that. I now have 54" snug.
I subtract the base line from the bust line, and that gives me 4"
Now use the chart to calculate cup size:
1"= A
2"= B
3"= C
4"= D
(And so on)
My bra size is base line/cup, or a 50D.
This is just a starting point as all the bra companies seem to have their own definition of what cup sizes are and will fit differently. Even different colors from the same company will change slightly.
Going in and trying them on is the only true way of selecting the correct bra. But the measurement will give you a starting point as to where too look.