I always had the understanding of measuring ones self as rather easy.
Use a seamstress tape measurer and measure around the fullest part of the breast, (usually directly across the nipples), holding tape snug, but not tight.
The results is the inches of size in diameter of the chest. The cup size, however..not really certain how to get an accurate measurement on that, other than to try on various bras to find out?

I was a dink size A cup...34 A (possibly a "sub" A....lol)...although the numeric means only the measurement around the chest. I think its the cup that makes more the difference....not really certain, though.

I used to shop in the teens section for myself for a long time, and to hell with anyone who may have looked at me. I'm old enough to have a teen daughter, (who happened to have quickly exceeded my breast size before she even reached 13...the brat...lmao), and shopping in teen's section wasn't an issue, (cept for maybe the fact that I hated the idea that I HAD to for myself....not for daughter).

I never developed breasts, which for a long time I didn't understand. My sisters had developed just fine....but not me. I had read sometime ago that smoking marijuana at an early age (can) deplete the estrogen development in some girls. Anything's possible, I guess. I figured that it's just my luck that I'd be that one who'd fall into that category to be effected by the use of marijuana.
I began smoking weed heavily at 13 and didn't stop til my mid 20's.
Finally got a tit job anyway. Lived most my life being teased for being "half a woman", and it wasn't until the psychological downgrading by ex hub that finally made me feel so ugly that I had a job done. I must say, though, I DO feel like a "complete" woman, now...(although I would have been just fine the remainder of my years without the surgery IF ONLY I hadn't allowed my ex to effect me like he had).
No matter. I feel wonderful now....and he's not around anymore to even enjoy the "improvements"..lol...the ***, anyway.

Shangrala