To get a good grasp of the adjustment, I'm showing you below the outline of the new pattern we will get, once the adjustment is done, above the original pattern outline. The new outline features a longer and wider bodice, especially at the front. To streamline the front and back, the back will also be lengthened, but not as much as the front.
The full bust adjustment is made using the cup size, because it determines the gap between the fullest part of the bust and the "band size", namely, the ribcage, just below the bust. Refer to the chart below to determine your cup size.
Since our patterns are adjusted according to a B cup, a full bust adjustment will be needed to all cup sizes above.
CHOOSE YOUR SIZE
Choose your size according to your band size, as shown on the chart below, instead of your bust girth. Between two sizes, pick the larger one! Also, be sure to widen or taper the pattern at the waist and hips level when your measurements differ from the body measurement chart provided with the pattern.
HOW MUCH TO ADD?
Simply remember the difference of 25mm (1") between each size! For instance, you will need to widen the pattern by 25mm (1") at the bust for a C cup. When making the adjustment, the length of the torso will also increase organically. However, up to you to determine if you wish to add more length, either by referring to previous sewing projects or by making a toile.
Have to hand a roll of kraft paper or patternmaking pattern, scotch tape, a square ruler, a ruler, paper scissors, a pencil and your pattern, cut and adjusted to your measurements and height.
Personally, I prefer to use the kraft paper rolls, because they are cheaper and are easier to find. You will find them at the post office, at the stationary or at the craft supply shop. Note that you will need a roll measuring at least 70cm (27") in width.
Trace a second dress (1) flipped to the other side. Your first dress will be for the front and the second, for the back. Then, cut the front along the front neckline and the back, along the back neckline (the front neckline is more scooped than the back).
On the front dress, extend the notch in the middle of the neckline toward the left side. This line refers to the center front. Then, trace a line perpendicular to the center front by starting on the right side, at 7cm (2 ¾") below the armhole. To indicate the apex, mark the middle of the line and report the marking to the other side of the center front. The apex corresponds to the highest point of the bust.
Trace a line to connect the armhole notch and the right-side apex. Then, square a line from the left side to the left-side apex. Square a line across the pattern, from the left side, at the hips level. From this hip line, tracer a vertical line to each apex.
By following the lines made previously, slash the pattern on the hip line. Then, slash the vertical line from the hip to the right-side apex, and finish just to the armhole notch. Cut also from the right side just to the apex.
At this stage, you can tape the pattern over another piece of paper, leaving enough room in the bottom to tape the skirt. Tape in place the shoulders and the neckline.
Rotate the right side toward the outside to obtain a gap in the apex corresponding to half the measure determined earlier. Tape in place the armhole/right side corner. Then, rotate the lower part of the right side toward the inside, until the vertical opening is parallel. Tape the right side in place. You now have a bust dart on the right side.
Note the width of the vertical opening.
By following the lines made previously, slash the pattern on the remaining vertical line, from the hip to the left-side apex, then cut in a corner to end to the left side. Tape in place the corner armhole/left side.
Shift the left side cut-out toward the outside to create a gap in the vertical opening matching with the other side (see the previous step). Then, shift the cut-out toward the bottom, so that the hip line is matching with the right side. Tape the cut-out in place.
Trace a vertical line starting from the armhole/left side corner, down to about 60cm (23 ½") below the hip line. Be sure that the line is perpendicular to the hip line, and parallel to the grainline.
Tape the lower part of the dress to the bodice, edge-to-edge at the hip lines, and the left side aligned with the vertical line.
Reduce the bust dart created on the right side by half and reshape the legs (see the dashed lines in the illustration below).
On the back pattern, square a line from the left side to somewhere between the armhole and waist notch, and slash along this line. Then, tape the lower part of the back pattern to the front by matching the hemlines. Do not align the back over the traced vertical line, but with the cut-out on the left side (the front and back must not overlap).
Before you trace the new outline, lengthen the back torso to streamline the front and back. To do so, tape the upper part of the back pattern by creating a gap in the slashed line corresponding to the reduced dart intake. If the paper below is not wide enough, tape a strip of paper to fill in the gap.
Trace the new outline of the pattern. Then, add a notch at 4cm (1 ½") from each side of the dart leg. Be sure that you measure starting from the reduced dart legs!
On the front right side, measure the distance between the armhole and the first notch (X) and between the waist notch and the second notch (Y). Report these measurements on the back right side.
Finally, true your pattern by checking the distance between the waist and hip notches, and hemlines and reposition the notches if needed. Retrace the grainline and the outline of the pattern.
Before you sew using your actual fabric, I recommend you sewing a toile to be sure that the fit is right for you. When you sew the side seams together, be sure to stretch the back to match up the corresponding notches at the bust level.