How do you get fitted for a bra?

They measure your bust-- that's from the widest part of your chest all the way around. Then they measure just below your breasts. The number in inches that they get for the second number is your band size (the number in bra sizes, like 29A or 36D-- 29 and 38 are examples of your band measurement).

What they then do is subtract your band size from your bust size. The difference is what determines your cup size (AA, A, B, C, D, DD, etc.) I'm not sure on the numbers exactly, but I know that cup sizes tend only to be an inch or two apart from each other, so (estimating here) if there's, for example, a six inch difference between your bust and your band size, then you're an A cup, but if there's a seven inch difference, you're a B cup, eight inch difference, C cup, and so on and so forth.

EDIT: Keep in mind that these are AMERICAN sizings that I'm using. link that helps explain the conversions between measuring your bra size in centimeters and inches.

they put a measuring tape around you

In the UK Back sizes go from a 28 inch back up to 40 something. You measure just below you bust around your rib-cage. Measurements go in 2s so 28, 30, 32, 34 ...etc. If you are in between measurements you go to the bigger number, I think.

I'm not sure how they measure the cups, people who fit you can usually tell according to your size (figure) and what size your already wearing. I do know that different back sizes changes the cup letter, but essentially are the same size, so a 28DD, 30D, 32C 34B and 36A are all the same size cup but a different back size, don't ask me why we do that because I think its stupid and unnecessary but they do.

The band of your bra should be in an even line around your back, i.e. not rising up the back or too far down, if it is unlevel then it means its the wrong size. The cup should run smoothly across the breast, i.e. no bulge(too small) or gap(too big).

In the UK the best places to get fitted are probably Bravissimo, La Senza and Marks and Spencer's. Although Bravissimo is the only shop that goes down to a 28 back(I would know I've trawled up and down the high street looking for a bra that fits and they're the only shop that stokes bras that fit me). La senza starts at 30. M&S occasionally does 30 back but not in much range.

Obviously Cinders knows more, but I hope I still helped.

by trying it out...it depends on the size of your
boobs!;)