How to Make Godets (Chiffon Inserts)
I had come across godets before as a flamenco dancer, but in flamenco they're usually made of the same type of material as the rest of the dress. Also, flamenco godets have a different function - they're designed to make the whole hem flip, whereas in belly dance, the skirt is fairly static and it's just the chiffon that wafts and floats. For that reason, in belly dance we normally make the inserts much bigger: not just a triangle but a quarter-circle, or even a half-circle.
You can insert the chiffon into a seam, or you can cut a slit in the fabric to insert it. If you cut a slit, be sure to sew across the top of the slit several times to strengthen it: that area will be under pressure every time you take a step, and the constant tugging can easily tear the slit further.
Measure from the top of the opening to the hem and add a hem allowance. That's the length of the edge of your quarter-circle. If you're using a semi-circle, multiply it by two.
The semi-circular godet is the easiest to sew in, because you can start at the hem on one side, and just keep sewing up one side, across the top and down the other until you get to the other hem. It doesn't matter if you don't have it perfectly symmetrical, because you're going to have to even up the hem anyway.
With a quarter-circle you must get the point of the godet positioned exactly in the centre of the slit at the top, then sew down from there to the hem on each side. If you get the point off-centre your insert will bulge instead of hanging nicely.
Once you've sewn in the inserts, leave the skirt to hang overnight.
This is where things get difficult, because even if the material doesn't "drop", the hem of your godets will need levelling: the centre will be hanging down far below the edges.
The easiest way to get the hem right is to wear the skirt and get someone else to pin it in place for you.
The next best thing is to lay your skirt on a flat surface, spread out the godet and pin the hem to line up with the rest of the skirt - then try it on, adjust as necessary, over and over again until you get it looking right.
I'm sure there must be a way to cut the chiffon properly in the first place, so you're not faced with this difficult hemming challenge. Once you're finished, the hem of your godet will still have a curve, but it will be much shallower. I've tried cutting it into a shallower curve before sewing, but I wound up with a hem that was shorter in the middle than on the sides (still scratching my head over that ...). Maybe a more experienced dressmaker can enlighten us!