I have been meaning to do this and so I will do it now. I made several flat pattern adjustments to my first shirt based on what I have learned to do to make a standard pattern fit me.
These are my usual alterations:
1. I add 3" to the length. I am tall and for some reason patterns seem to be running shorter lately. I can always trim down if it is too much.
2. My fit has improved 100% since I started buying patterns by upper bust and adding to the side seams, with Nancy Zeiman's pivot and slide method which preserves the smaller armhole (my arms are kind of scrawny, unlike my legs) rather than the usual method of adding to the side seams which increases the armcythe. I add 1/2" to each side seam (2" in total) and also make a full bust alteration, not because I have large boobs but because I am larger in relation to my upper bust measurement. I was excited that this was a multi-sized pattern so I was saved all that cutting and slashing stuff.
But thanks so much to KMQ at Smoking Needles for alerting me to the fact that there were some things to check out with princess seams, because she saved me some trouble by running into first herself.
I know that a princess seam fit requires a measurement from centre front to the seam line, which is supposed to run down the apex of the bust, and from the shoulder neck intersection to the fullest part of the curve. In this case I found both measurements were a little skimpy on me which kind of makes sense since a larger bust may be lower and wider than the norm. I did about a 1/4" and 1/2" realignment to the seam line position and widest part of the curve respectively and I think that is going to make a difference to my fit.
3. I rotated the shoulder seam up 1/2" at the outside edge to accommodate square shoulders, again Nancy's method. Basically you cut out the neckline, put a pin at the intersection of the neck and shoulder seams, rotate up (or down) moving the outer edge of the shoulder seam, cut along the new shoulder, move a pin to the intersection of the outer shoulder seam and armhole seam, rotate the pattern back so it aligns back so the centre fronts match, and cut out the armhole and down. This will also be adding, or subtracting, a little to the bottom of the pattern. And do it both front and back of course.
This shoulder adjustment is really really important and one of my most important alterations. It is also not something you see mentioned a lot.
For years I avoided shirts because no matter what else I did the necks rode up. I finally figured out, by looking at the pattern, that there was a slope down to the shoulder seam and my body went straight across, like a ruler was on my shoulders. There was no where for that slope to go except up around my neck.
This makes sense when you think about it, that shoulder fit is crucial, because the whole garment hangs on your shoulders.
For a totally fascinating description, beautifully illustrated as usual, of the exact opposite feature, sloping shoulders, which I expect more women have, go to my friend Robin's blog .
I really think that looking at shoulder fit is The Next Big Thing and might even be as big as FBA was when it hit our world.