Run a line of basting stitches along the seamline on the larger piece of fabric (you can see the larger lime green fabric will be gathered onto the smaller avocado green stay in the picture below). You can do this by machine by cranking up your stitch length as high as it will go - on my machine, it is 5 mm.
Pin the two ends in place and pin the middles of the two fabrics together.
Continue to pin at equidistant intervals.
Add as many pins as is necessary to evenly distribute the fabric.
Pull up the basting stitches to gather the larger piece of fabric as tightly as possible.
Pull the ends of the smaller piece of fabric back into shape. The gathers on the larger piece of fabric should now be evenly distributed amongst the pins. Sit back and revel in your awesomeness.
P.S. The dress is done and not a moment too soon. I will be heading out in a couple of hours. Hopefully tomorrow I will have all sorts of fab photos of me rockin' out at Prom.