how the hell do u measure breast

i swear to god its been 3 years and i still cant do it right. ive been told what i am and google images confirmed it. but when i do the measurements i got DDD?! what is the meaning of this. im 100% sure i did the measurements correctly but what the hell did i do wrong. (i used inches, followed tutorials, googled stuff and everything the world had to offer but this damn monke cant get it right)
although i already know my breast size i just want to get the actual measurements.