(This is the FFRP slash/spread method.) If your bra cup size is more than B, pick the pattern sized based on your UPPER BUST measurement as the bust measurement. By choosing your size based on bone structure rather than fluff, your pattern will fit through the shoulders and upper chest with minimal upper body alteration, if any, necessary. It's easier to adjust patterns for a full bust than to grade down for a proper upper body fit.
These illustrations show an armhole princess bodice. The alteration is the same for a shoulder princess, just adjust the armhole slash point accordingly.
Slash the side front bodice, as shown below in pink, leaving pivot points at the armhole and bust point.
If using a pattern with seam allowances included, the armhole pivot point should be at the sewing line, not the cutting line.
Spread the slashes apart the distance needed to meet the pattern center front to your center front (per your initial tissue-fitting), or the difference between your full bust measurement and pattern full bust measurement (per envelope), keeping the vertical and horizontal spreads parallel to the slash lines.
A full bust alteration will add length under the bust. Move bottom center piece straight downward to match pattern bottom.
See animation below.
Add tissue behind pattern to fill in, leaving the side dart unfilled.
Slash the pattern piece as shown in blue near the bust point.
Rotate the unfilled side dart until it's closed and lays flat on your work surface. This will create a dart opening at the princess seam.
Measure the two openings shown between the two sets of blue slash lines and note these measurements for adjusting the center front bodice piece, explained below.
Fill the new opening with tissue behind the pattern and true the princess seam.
Create two horizontal slash lines on the center front bodice, in roughly the same area as the final dart rotation and length adjustment made for the side bodice piece above.
Spread the pattern vertically, in two steps, the same distance as the measurements you noted above.
The armhole remains the same length so no adjustment to sleeve is required.
Fill the spreads with tissue behind the pattern and true the seams, if necessary.