Let’s start softly by drawing the seam allowances of pieces n°1, 2 and 3 of the front bodice. Be careful, because some areas don’t include seam allowance (highlighted in orange below)!
Cut the seam allowances that you’ve just traced. Don’t forget to copy the notches before cutting!
Note the amount of fabric that you need to remove from the bust on one of sides of the bodice (for example, 2cm).
On piece n°2 (front bodice middle), measure half this amount (here, 1cm) starting at the notch. With the help of a curved ruler, retrace the bust line using your mark as a reference, as in the photo above.
Do the same with piece n°1 (front bodice top), and cut the two pieces following your new lines.
Lay down your bodice pieces, carefully matching the seam lines (for the bust line, match the beginning of the curve as in the photo above). You can now see that the center of the bodice is not straight, rather slightly pointy. We need to correct this!
Carefully tape the pieces on a piece of paper, and draw a line joining the top and the bottom of the bodice.
Detach pieces n°1 and 3: just cut them along the line that you’ve just drawn.
Once you cut the pieces, correct the grainline by retracing it parallel to the edge (the new grainline is in orange in the photo).
Finally, let’s modify the middle piece in order to have a nice point at the center of the x motif. Start by tracing a line perpendicular to the first line you traced, passing through the point of the piece.
Cut the piece along this line. On the tip of the point, you can see two small lines corresponding to your first line. Don’t erase them, they’re very important!
Overlap the two pieces keeping them parallel, until you match the two small lines as in the photo above. Cut the two small tips to form the point of the piece.
After cutting, correct the grainline as you did with the other two pieces. Verify that all the seams are matching and are the same length… We’re almost finished!
Add the seam allowance to your new pieces. Remember not to add it to the neckline and armholes.
Finally, you will need to modify the back bodice, because the SBA slightly shortened the front. Draw a line perpendicular to the grainline along piece n°4 (back bodice), and cut along this line.
Shorten the back by the same amount you shortened your bodice middle (piece n°2). And voila, your SBA is finished!