I prepared a rectangle and ironed it like this, then sewed the narrower half, right side to the right side. Folded the bottom, the center of the triangles is not equal to the one of the halves. I folded the piece according to the ironing and sewed neatly at the right line so that the stitches almost wouldn't be visible (it attaches the back side of the strip as it's a bit wider than the front side). The same for the other piece. Finishing the triangel is a matter of hand sewing.
Today I bought metal eyelets. I'd like them to be covered with a cotton - as if sewed in hand but stronger. If I have time once, cause now I'm in a hurry. The 12th is the ball I want this dress for:) There won't be this nasty black something, I want a fingerloop cord there instead.
Cottesimple was right, this picture was taken without a bra and it seems like a good support.
In the meantime I've sewn the sleeves together and added two gores to the sides (too little yet). And today I undertook a neckline.