I used this method when making my Deer and Doe Myosotis Dress which has gathering on the sleeves, waist and a ruffled tier at the hem. It was very successful, even for the 2.6 metre hem ruffle. No gathering in stages, pulling (breaking) threads or having to distribute the fabric. It produced really tight even gathers with no hassle!
The method uses two rows of gathered stitches using shirring elastic in the bobbin. I had previously used the Stitch Sisters Youtube tutorial on How To Use Shirring Elastic.
You will need to use ordinary thread in the top spool and shirring elastic in the bobbin. It is easier to remove at the end if you use a contrasting colour, but not essential. Increase the stitch length to at least 4mm.
Firstly, take note of the seam allowance for joining your gathered and non gathered pieces. You then need to stitch, using the shirring elastic approximately 0.5cm either side of this measurement.
My seam allowance to was 1.5cm, so the first row I stitched with a 1cm seam allowance and the second a 2cm seam allowance. During the second line of stitching you will need to gently pull out the existing gathers in order to sew over flat fabric.
Then anchor the ruffled piece to the plain piece with pins, starting at the side seams and centres.
Then just stretch and pin until the gathered piece fits with the non gathered piece.
The gathers loosen but in a very even fashion. Now its time to join with stitching, so change the bobbin thread from elastic to matching thread, along with top spool if that is contrasting. (I forgot to do this and sewed up with elastic the first, time!) Also reduce the stitch length.
Now you need to sew between the lines of elastic stitches you have just made, at your given seam allowance. This is where the gathers are neat and tight.
Then check your work. It may appear a little over gathered until you remove the elastic stitching. So, if you are happy, do this. Pull the elastic out, (in one go if you are lucky) then the thread will be released and easily cleared away. All that remains is to finish and press seam allowances.
I hope you find this as useful as I did. I really struggle with pulling threads and evenly distributing the gathers. This seems so easy in comparison and gives superb results.