I am 5.11, 38 chest and mine are DD cups, so in theory a 38dd bra size. I got this size as I was a bit overweight, when I got them so they fitted my size. My old ones were a 36C (which fits better and looks better on my shape now). The products I have can be used in three different options, I can wear them like a bra as they can have straps attached to them, our in a form-bra (slide in pouch), and lastly with adhesive (no straps).
In order to wear mine I line up the nipple on the form and my nipple then drop the form a cm lower to give the impression of "gravity". Therefore if I wore a bra it would sit in it perfectly as the bra gives it a bit of lift. However if you go without a bra, I line my nipple to the nipple of the forms. I hope that makes sense?
I wore them a few times at home without the bra and they look very real, but I heard they are easy to damage so wear a bra that fits nicely.