|For me, making a FBA with princess seams is much easier than with darts! As we all know, mainstream patterns are cut for a B cup - I think I last saw that in highschoo! No wonder it was so easy to sew then!
Anyway, you need to adjust for width first. There are a couple of ways to do this, but I like to use pivot and slide. This works especially well if you have sleeves because you do not need to change the sleeve You can see this method in Nancy Zeiman's book Fitting Finesse.
The easier, quicker way to do it, which works great if you don't have sleeves, is to just follow the armscye seam out farther to the size you need. This method also works great if you are small in the upper chest and need the armscye cut in toward the CF more anyway.
Now that you have the width of the bust adjusted, it�s time to go on to the next step.
This is the easy part. Cut the side front pattern piece 1" below the armscye seam from the front seam to the side seam, but not through it.
Then, spread the pattern 1/2" for each cup size above a B. Spread it at the seam line, not the cutting line. So, for a D, you would spread it 1" at the seam that meets the front section.
Draw on the new curve on the cutting line either freehand or using a curved ruler.
Now, with the side of your tape measure, measure the length of the line you just drew. It should be a little bit longer that the amount you spread it. Using this measurement, spread the center front pattern piece the same amount. (see photo link above)
Make sure you are spreading it at the same place as you spread the side, by lining up the neckline or arm seam, depending on which kind of princess seam you have.
This method works for princess seams that go into the shoulder or the armscye.
As with any time that you need to make adjustments to a pattern, be sure and make a muslin first. At the very least, cut out your pattern with 1" seam allowances and baste to check the fit. It is much easier to adjust the fit on a princess seamed blouse/dress because of the extra seams..