I found out that not only canvas, but even this fabric is the most stretchy diagonally. I fulfilled the bra with my sweater and started to strain. First, I pinned the two spots on the vertical axis, then the sides. Having this part of the work succesfully done was a great relief.
I used to use backstitch, even invisible backstitch when necessary, but now I prefer other things. I drew a line at the base, along the whalebone. Then I pricked the needle upwards to this place, arrange the cup's fabric and brought the needle down through it (very, very small stitch). In fact, this was quite pleasant:) I don't mind the pattern the stitches make.
Well, the worst is over. Only bead embroidery's in front of me and I'm looking forward to it!
You may see that the center isn't really symmetrical and neat. That's because I didn't want to spend much money on a bra that wouldn't be visible and I which I would use only couple of times a life. Cheap bra and luxurious fabric seems better to me than the contrary.