So the typical instructions for making gathers are to sew 2 lines of basting stitches and then gently pull until the fabric gathers. PITB if you ask me. Especially when there's a lot to gather. Here's an easier, faster, no stress way.
You will need:
*Crochet thread or thin yarn
*Zig Zag foot -or even better some kind of foot that you can feed the yarn into, I used my Satin Stitch Foot (it's the clear one)
Set your zig zag stitch on it's widest and longest setting. Lay your fabric down. Position the yarn in the middle of the zig zag foot. This is where one of those other feet come in handy. It eliminates all the guess work.
Start zig zagging over the yarn, without catching it in the stitch. This what it will look like when you are done. (click to enlarge) Pin the edges of the yarn down. I had 2 yards of material to gather, so I split it in two, sewing the yarn onto each separate yard, that's why there's 2 pins.
Start pulling the yarn on the other end, until the fabric reaches the width that you need and pin the other end of the yarn down.
Now pin the gathered fabric onto whatever you're sewing it onto, right sides together. In my case, it was a t-shirt. Make sure the top of the fabric is pulled neatly up, so that it doesn't get caught and peek out at you from the right side.
Side note: If you want to sew this onto a t-shirt to make a dress, the neck of the shirt will be at the hem of the skirt when you're pinning.
Position the needle to the left of the zig zag stiches. This is important! If you position it on the right of the stitches, the right side of the garment will look like this (click to see the riff raff) and then you'll have to spend time pulling out the zig zags. Which will defeat the purpose of trying to do this thing faster.
Now just sew the gathered fabric onto your other fabric using a straight stitch. In this picture you'll see my original sewing line (on the no-no right side of the zig zags). Again, make sure the gathered fabric is laying straight up.
In my house, it's best to use the Straight Stretch Stitch (marked with an S.S. on my machine). It keeps better for active little girls. When that's done, pull out the yarn. It comes out quite easily.
Viola! Easy Gathers.
Hope this was helpful and easy to understand. If you need anything clarified, let me know.