There is some futzing about that happens before you begin throwing fabric at the body you're making a sloper for. The form gets a little preparation. (People do, too, but it's not quite the same; since I'm not draping on a human, I'm not going to get into it since I have no hands-on experience with it.) I know I already described and posted photos of this, but here it is again, in context.
The form gets a line of tape parallel to the floor at the bust and the hip; at the bust, the drape goes over the stretched tape - called a bust bridge - to make sure the garment fits properly. This bridge might not be used for things that fit tight (e.g. a bustier or a strapless ballgown), but for a fitting block we definitely want it. It also gets pins to make it easy to locate key points by feel - the tips of the shoulders, at the level of the screwhole along the armscye, the armhole depth, just below the center neck, the bust points, and if you like the center front along the hipline. At this point I also took a series of detailed measurements; I was pleased to find that Marie's manufacturer specs were right dead on the money.
|Marie looks lovely in black.|
|Detail showing pins at neck, bust point, and armhole.|
The muslin is sized based on the form measurements - full torso length x chest arc (from center front at the bustline, on top of the bridge, to the side seam) plus four inches in both length and width; the back is the same, except with the back measurements, of course. Selvage edges should be removed and the grain corrected if your fabric is off grain, which muslin often is. Then the center front on the muslin has a 1" allowance folded and pressed, and a temporary neckline cut in to help get things to lie flat as you start. It's also worth noting that you only drape one side of the body, so the muslin should be prepared accordingly; I am choosing to do the left side (mostly because I'm left-handed).
|The prepared muslin.|
Now - finally - the muslin for the front goes on the form! Pin it at the center front neck, center front waist, and to the tape at the center bust. The horizontal crossgrain line should line up to the bustline, naturally. Carefully smooth the fabric over, and pin at the bust point.
|Finally, Marie sees fabric. And pins. I don't envy her.|
|The neck and center front waistline|
Smooth the fabric around the side of the form to just past the center seam; since this is a one dart bodice and all the dart intake will be at the waist, the crossgrain line should be sloping down at the side seam. Pin at the armhole depth pin on the form. Smooth the muslin down the side seam and pin at the waist tape. There should be a fairly large fold of fabric sticking out below the bust point; this will become the dart intake. Mark the side seam on the muslin; most forms have hand-stitched side seams that stand up a bit, so you can make a rubbing with the side of a pencil tip. Remove the pin from the armhole ease tuck. Measure 1/2" back from the armhole pin and mark; draw a new side seam from the mark to the side waist. This also adds wearing ease.
|Now is a good time to trim away some of the excess fabric at the top, bottom, and armhole.|
Turn your seam allowances under at the side and shoulder seam; finger press them, and repin, being careful to use the redrawn side seam. FINIS! Or well, the draping portion for the front is finished, at least. There are a bunch more things to do before we're actually done.
|The completed fabric drape for a one dart front bodice.|
It's a lot of work to do it right, but it's worth it.