how to draft a gathered dart (using a pattern you already have)

Gathered darts regularly feature on vintage garments.  There's a simple way to get this effect, using a pattern you probably already have in your stash. 

 You can use any basic skirt pattern that has two darts on the front, ideally a pencil (shown here) or A-line.  If you don't already have something appropriate you could use something like Simplicity 5259 or possibly New Look 6843.  However, pick something where the darts are prominent, by both their position and length.  Or, experiment with another shape if you like! The effect will be different but the principle is the same. 

You could also use a dress pattern as your starting point, where the skirt has two front darts.  Just trace off the skirt and add a waistband.  Burda's Little Black Dress 10/2011 would probably work.  

 I used my block, below

First, measure the length of the dart furthest from the centre front.  In my case it's 12 centimetres.  Divide the length of the dart leg by four. 

 Then draw four lines across at these points, at a right angle from the centre front.

Since I'm using a block, I added my seam allowance - along the side seam and the waist only - now. 

Now cut out the dart.  Then cut across each of the lines you just drew, to the edge of the seam allowance.

You may want to add letters to each of the segments (in case there is a sudden gust of wind!) 

Now here's the bit of math: My dart leg was originally 12cm and I'm going to double it to 24cm.  That means I have to add 3cm between each 3cm strip to make 24cm total.  Keep in mind, this is not a science, as long as your strips are spread evenly you will achieve a nice gather, whether the dart leg ends up being 22cm or 26cm.

(NB:  For each version of the skirt I made - lovely wool gabardine from fabulous Truro Fabrics and a light cotton, shown below - doubling made sense given the weight of the fabric.  If you are using heavier fabric, consider how much the fabric will "bunch" if doubled. You may want increase the dart leg by only 1.5 instead.  But this is only a guideline and not a rule...!)

Once you've decided on the placement, pin the strips in place.

Now glue the strips in place.  (Or use tape, like me.)

Then add a seam allowance to the dart itself.  I made mine 0.5cm to reduce bulk.

Once you've cut out the fabric, gather the long edge of the dart by hand, to match the length...

...use pins to keep in place, and sew!

The summery cotton version: