Slash and Spread 101
We designers talk dirty once in a while, especially when it comes to alterations. Slash and spread is an easy way to add fullness for shirring, pleats, gathers or darts. I often make a muslin (a mock-up in cheap fabric) before committing a design to real fabrics in case I want to make changes to the original design or fit. Example: “My Better Half” dress for Garrott Spring 2011. The pattern did not have the desired shirring on the knit side seam as the sketch shows. I added fullness in until I got the desired drape and effect you see in the finished garment.
My Better Half Sketch
Slash and Spread: I begin by slashing the pattern in 7 lines from the place where I want to add fullness to the place where I want the drape to fall from. I am careful to cut up to-but not through the end of the paper. I want the pattern to remain in one piece and not to alter the shaping on the other side.
Dress Lower Front Slash Placement
I space the lines closer together through the hip and farther apart at the bust because I want most of the fullness to drape at the hip. Red arrows indicate which side to spread each cut. Adding 6 inches total along this side seam between the notches, I tape it down to a new piece of paper and re-trace the new pattern. Be sure to transfer all notches, and marks to the paper below. 
Pinning and draping until it looks right
It may take more than one go to find the right ratio, I cut this piece twice to get it perfect. I pin the shape to the dress form with the previous muslin under it to compare the fullness.
Back to the cutting table
Work and play
“I love it when a plan comes together” and how fun is it to put the label on it when it does.
Finishing touch
“My Better Half”
