Gaylen asked for some tips on sewing Challis; I'm not going to pretend to be an expert but I'll share what I've learned...
For sewing rayon in general, check Shannon Gifford's sewing guide at Emma OneSock . I thought she had written a guide for challis, but I didn't find one. So here's my two cents' worth.
First, make sure you pick a pattern that is compatible with an uber drapey fabric. Think loose, blousey, soft. This is not a fabric from which to make a pencil skirt. ;-)
Challis shrinks; be sure to pretreat it. Sometimes the dyes are not stable; I ruined a very nice new top by putting it the same cold water wash as a new piece of (admittedly cheap) rayon challis. So wash it cold by itself. I usually run it through the dryer, too, the first time, although I will drip dry the finished garments. Rayon feels very stiff and woody when it's wet; don't panic, that's a function of the fabric (also a way to know it's really rayon...). It will soften again when it's dry.
Actually constructing the garment is an exercise in patience; challis can't be rushed.
Challis is very unstable, which means it is a PITN to cut. A helpful hint here: spray starch. That will help stabilize the fabric so that it doesn't slide off grain or distort. Press and starch the fabric immediately before laying on the table. Also using a cutting mat and rotary cutter will help lessen the distortion when cutting out the pattern.
Once the pieces are cut, handle them as little as possible to keep them from distorting. The fabric is fragile at this point. It will ravel; zigzag or serge the seam allowances after a seam is sewn.
(This is important!) Mind the direction of the stitching; always sew from the wide end to the narrow end; sew slowly and avoid pulling on the fabric as much as possible. I don't pin much, but when I'm sewing challis I will use pins to make sure the seams feed evenly and don't shift. Pin on the seam line (put the pin in perpendicular to the seam; in and out of the fabric right where the seam will cross). Hand basting can also be your friend here.
If your garment is a bias cut, allow extra seam allowance width and sew with a very narrow zig-zag
I know all of this sounds very tedious and picky, but a well-sewn challis garment falls beautifully and feels very feminine. Be careful and don't hurry and you should be fine.
I have a gored rayon skirt with godets that I thought I could sew right up; the gores all went whopperjawed and none of the seams were the same length (those bias edges really distorted). I unpicked the whole thing, pressed it carefully back into shape, and restitched, paying attention to direction and such and it actually fit together and is one of my favorite summer skirts. Wish I'd been more careful the first time...
So, there's my limited wisdom on sewing challis. If anyone else has any tips or suggestions, please leave them (or links to them...) in the comments! Gaylen and I will both appreciate it. ;-)