Problem: Fit a square peg in a round hole. That, in essence, is what you are doing when you make a shift. You need to make a group of rectangles fit a three dimensional curvy body. Making a shift is a great example of very clever geometry!
Here's a diagram to help explain it. First you need to take a rectangle and make it shaped more like a body -- narrower on top than on bottom. You do this by slicing off two long triangles at the top, flipping upside down them and attaching them to the bottom - creating gores. Two rectangles become the sleeves that are made wider at the top than at the bottom by the use of two small squares that become underarm gussets. And Abracadabra! You have a garment of squares and triangles that turn into a shift.
Your humble & obedient servant,