Determining your size should be relatively straightforward. The first thing you have to do is measure yourself.
Measuring is an important step and I recommend doing this every time you make a pattern, in case of any weight fluctuations you may not be aware of. Different pattern companies also use vastly different sizing, and give the sizes different numbers (which are also completely different from the high street sizes) so definitely, definitely measure yourself and pick your size based on that and nothing else.
Ideally, you should ask a friend to measure you when you’re wearing the underwear you plan to wear with the finished garment. Stand up straight and breathe in. Place the measuring tape around the body so that it’s parallel to the floor. Don’t stretch the tape or squeeze it too tightly around yourself. The bust measurement is taken at the fullest point of the bust, the waist measurement at the narrowest part of the torso, and the hip at the fullest part of the hip. Make a note of the measurements and compare them to the size chart to determine your size.
Pick the size that is closest to your measurements. If in doubt, choose the larger size – while it’s possible to take garments in, it’s much harder to let them out if it comes out too small.
Lengthen/shorten here lines
The finished length of the blouse is between 60cm (for the size 10) and 63cm (for the size 18), measured from the top of the shoulder to the hem. You can get a rough idea of whether this will be right for you by measuring a blouse you already own, or measuring your body.
If you want to lengthen or shorten the blouse, here’s how to do it.
Identify the lengthen/shorten here line on the pattern (marked in red below, but on the pattern it’s in grey). Cut along the line and you should be left with two pieces. I’m only showing the blouse back here, but you’ll need to do the front and the front facing as well, of course.
If you’re intending to lengthen the blouse, first decide how much longer you want it to be. Then, place the pattern pieces on another piece of paper and glue them down, the required distance apart, as illustrated below.
You’ll need to redraw your lines to join up the shape. For the curved lines, you won’t be able to follow the lines exactly, so just draw in as smooth a curve as possible that starts and ends on the correct line for your size. Then repeat the process for the blouse front and front facing.
If you wanted to shorten your blouse, you’ll need to overlap the pieces by the amount you want to shorten the blouse by, and stick them together as illustrated below. Join up the curved side seam as smoothly as you can. Don’t forget to do the same thing to the blouse front and front facing.
Grading between sizes
Do your body measurements span more than one size on the size chart? If your measurements are one size apart (say, your bust measurement is closest to size 12 but your waist and hip measurements are closer to size 14), I’d recommend just cutting the larger size, as it’s rather a loose-fitting blouse anyway.
If your measurements span two or more sizes, you can draw a new shape for the blouse based on the existing. In the example below, I’ve drawn a new shape for someone who fits size 12 at the bust but is closer to the 16 at the waist and hip. Using a coloured pen, draw around the top part of the blouse, including the neckline, shoulder seam and armhole at size 12. The bottom of the armhole is roughly at the bust level, so stop there. The waist level is at the narrowest point of the blouse. Draw a dot or make a mark on the pattern at the waist on the size 16 line. Now draw a new curve starting at the bottom of the size 12 armhole, then joining to the mark you made at the size 16 waist, and following the 16 line all the way around the bottom half of the blouse. Try to make the line as smooth a curve as possible so you don’t end up with a lumpy blouse.
Remember the top part of the blouse is size 12, so cut the collar pieces to size 12 too.
Making a toile/muslin
A toile or muslin is a test version of the garment made in a cheaper fabric that has similar properties to the final fashion fabric, that is, it's about the same weight, drapes in a similar way, etc. For this blouse, I recommend making a toile if you're worried about fitting, especially if you find your figure difficult to fit or are planning on making major changes to pattern as drafted (eg FBA).
For the toile, cut the front and back pieces (and one collar piece if you adjusted that) and quickly sew the side seams and shoulder seams together with a 1.5cm seam allowance, to check the fit . Mark the buttons and buttonhole locations on the fronts (don't forget, there is a seam allowance up both sides of the centre front... use the marks on the pattern to get the right location) and just pin your blouse together while you're wearing it, with the button/buttonhole marks matching. Check the fit to see if the size is correct, and make any changes you need to make before cutting out your final fashion fabric.
If you're cutting a standard size with no modifications, or are just a bit of a risk taker at heart, you may choose to cut your final fabric without a toile, and of course, that's fine too. No judgement here! Just be aware of the risks of progressing without one ;-)
I hope that answers some of your questions on the most common types of pattern alterations. In the next couple of posts, I have two very talented guest bloggers who will be covering grading up your blouse beyond the size range, and how to do an FBA (full bust adjustment).