The best piece of advice I can give you is buy the Palmer Pletsch book Fit for Real People or the new edition The Complete Guide to Fitting. The first I can definitely recommend and you you can see the silly money prices the second hand ones go for. The new one I haven’t read, but is supposed to be a brought up to date version.
I always have to make a FBA and once you’ve mastered the concept you can adjust any pattern, although some do take some figuring out.
The basic principle is to make the fullest part of your bust fit the pattern, the fullest part indicated by the bust point. Most patterns still are drafted for a B cup and a twenty something. I can vaguely remember those days and yes I could sew from an unaltered size 12 Vogue pattern and get the perfect fit.
I am now a 14 back, 16 chest and FBA to size 18/20
The place to start is with your back, you may find like me my back is narrow in comparison to my front. Get some one to measure your back, side to side. The easiest way to do this is you hold a tape measure around your chest just under your arms, this means that it should be above your breasts, so measures body and not bust. The helper then places another tape measure over the top of the other and measures back side to side and then your chest side to side. These measurements determine the pattern size to choose.
Now pin the pattern at the shoulders and side seams. No way will this meet down the centre front. Make any adjustments the back my need. ie narrowing the shoulders, sway back, rounded back etc. Let me know if you need help here.
It’s amazing how adjusting the back can have an impact on the front.
Once you’re happy with the back make a note of the adjustments as you will probably need to make these on every future pattern. Re pin and try on, pulling close to your body and measure the gap from the edge of the pattern to your front, this is the amount of the FBA. Note where the fullest part of your bust is in comparison to the pattern. Now measure down from your shoulder where it meets your neck to your bust apex, ie nipple. Wear the bra you are going to wear with the top. I found different bras could make a difference of 2-3 cms in bust apex position. If the difference between you and the pattern is more than 3inches, make the bust apex alteration first, as anything less may be accommodated in the FBA or may end up being less. I must admit I have only ever done the repositioning down the centre front and not the side seam and done it after the FBA. The exception being a princess seam, but we’ll leave that for now. I draw a rectangle around the bust apex, say 4cm above and below an 2 cm to the left, the move down the required amount.
A dart can basically be anywhere around the edge of a pattern including the centre front if on a seam. If it’s a knit fabric and I don’t want a dart I will quite often split the darts around the side and ease and the bottom edge. Remembering they all have to point and be at least 1 inch from the bust apex. If you imagine icing a domed cake with a sheet of icing, it will fall in folds around the outside, you will then have to cut the fold/folds to allow it to mould around to give a smooth surface, it’s the same with our bodies moulding a flat piece of fabric round our lumps and bumps.
You will also find the curve of the arm syce looks strange. Don’t worry it opens out again when you close the dart.
For narrow shoulders line 1 would be perpendicular to CF or through waist dart if there is one and through bust apex and to the notch on arm syce, basically a third of the way up. Line 2 through the side bust dart to bust apex. Line 3 I usually cut along* the lengthen shorten line. A tip, place a large sheet of cardboard underneath the pattern after cutting but before adjusting and use pins to anchor the cut edges, stops them moving part when you’ve adjusted them. I just find them easier than weights.
After FBA you may also need to adjust the placement of waist dart.
Sorry if I’m teaching my grandmother to suck eggs, but I find a lot of instructions only tell you to make an adjustment on the front and to measure the difference between the chest and bust.
Hope this helps