Unless you are blessed with a ruffler foot for your machine, gathering stitches are the typical way of gathering ruffles: two rows of long-length stitches that you pull.
However, this is not the only option, if you have some thick, wirey thread (such as quilting thread).
This method of gathering is a lot more reliable than gathering stitches, but might require a little bit more clean-up. With this method, there is no risk of breaking thread, and if your gathering thread comes out, you can easily thread it back into place. Also, distriputing fabric is a lot easier to control and conduct.
First, prepare your ruffle fabric by finishing off both ends. Since I have a lot of fabric that I will be gathering down and I want a light hem, I choose to simply serge and turn to finish off the hem of my ruffle.
Now, set your machine to a zig-zag stitch, making it fairly wide and long. It is better to air on the bigger side if this is your first time using this method, so that if will be easier to control. The settings I used are above.
Now, divide and conquer! Divide your ruffle and your skirt hem into quarters. Pin quarters of skirt to ruffle Then divide those quarters into quarters.. You can keep going for more intensely accurate gathering.
At each quarter, use a pin to pull the quilting thread up for gathering. You can then gather to the appropriate length, and cut and tie the tails of the thread together to hold the gathers while you pin.
Stitch the whole thing down and Ta-Da! Correct any mistakes and press your seams open.
So excited to see this dress coming together! But before I finish it, I had better make a vest for bunny (not joking.).