|Adding bra cups to a sun dress or halter top is fairly simple to do. First, you must choose a lining for the bodice. I chose swim suit lining for this dress as it was comfortable and very stretchy (There are step by step pictures in the above link)
Next, lay the cups on the wrong side of the lining fabric. Leave 1" below the cups for an empire seam. You can also add elastic here if your dress does not have an empire seam. this will create a "shelf bra" Mark the cup placement with a water soluble pen or chalk before you pin.
Now pin the cups to the lining.
stitch around the cups with a zig zag stitch.
If you are using something very stretchy, you are finished! If not, cut the lining close to the stitching to remove it from the inside of the cups. Cover the stitching and raw edge with bias cut lining fabric or strips of tricot.
Lastly, attach this underlining with the bra cups to your bodice. If the bodice has a fitted back, this is all you need (believe me, I wear a DD and it is working for me.) If the bodice has a loose back, you may want to add a back strap of some kind to snug up the front.