For this tutorial, I have used a bodice pattern, that already has one dart in the front armhole. If the pattern you are adding darts to doesn't have any darts, you may want to distribute the front dart value into a waist dart and an armhole/shoulder dart for a nice fit.
Although I have used a bodice pattern, you could also use this technique for adding waist darts to a skirt or trouser pattern.
As well as removing excess width at the waist with darts, you may also want to remove some of the width by altering the side seam (by following this tutorial) as well. It is normally best to distribute the amount you are removing by combining the two techniques.
Mark in your new dart positions.
The front dart line needs to run from the point of the existing dart, down to the waist.
The back dart should be placed close to the centre of your pattern and can be the same length as your front waist dart (or a little longer).
I drew the line perpendicular to the waistline, but this is not necessary. You would prefer an angled waist dart.
To work out the width of your new darts, you will need to first work out the waistline measurement of your pattern.
To do this, measure your front and back waistlines, and add the lengths together, before multiplying the result by two (as we are working on half a pattern).
Decide on the amount you would like to remove from this result. In this example, I had decided that I wanted quite a close fitting garment. I took my waist measurement and then added 5cm (for ease). I subtracted this measurement from the waist measurement of the pattern, to work out how much distance needed to be removed by the waist darts.
Take the difference between your current waist measurement and your new waist measurement and divide the result by by 4 (as you will have 4 darts - 2 in the front and 2 in the back). If you are also removing some of the excess by altering the side seams, divide the result by 6 (treat each side seam as a dart too). This result is how wide each of your darts will be.
Distribute your dart width evenly, either side of the dart line, on both the front and back patterns.
Join the endpoints of these lines to the dart point, to create dart arms.