Another suggestion is a Hera marker. There are some tutorials on it and not expensive

Tailor's chalk is designed for use on fabric.
I've used pencil and it doesn't wash out.
Whatever you use, make sure you keep the package directions for how to remove all markings. There is one product that says ironing removes the marks, another product that says ironing makes it permanent. Be very careful to keep package directions with the products you use.

I use the Bohin Extra Fine Chalk Pencils. The line they make is like a pencil line and they completely washout and are refillable. You can get them on Amazon.

I use these as well. I like them better than the dritz chalk marking pencil as they are thinner lines. I only have a couple of colors - white for darks and I think pink (?) for lights. I've never had an issue with them washing out. The orange chalk in the dritz marking pencil set is equally as painful to remove as any yellow I've ever used. Most times for lights I use a #2 mechanical pencil. As long as I don't get too heavy-handed with my marking, those lines wash out. Sometimes need to use a soft toothbrush and dish soap to scrub out some heavy lines but they do ultimately wash out.

Peckish ... Does all tailor's chalk have wax in it? or just some?
.... as above, I haven't used, but am intrigued and wondering about its possibilities.

Just some. The wax kind is designed for use on wool fabric, such as men's suits. You hit it with an iron and it disappears into the fibers. If you use that kind on quilting cotton and hit it with an iron, it will leave a mark. The plain chalk kind doesn't leave marks that last very long, but the nice thing is the chalk simply brushes away.
The kind I like comes in powdered form with the little wheel, it makes a very fine line.

Thanks Peckish .... for the explanation about the wax type.
Moral of the story ... Buyer Beware and be sure to read!!
And then do ones own test trials to be sure!

I've found that General's Charcoal White pencils work better than chalk. I can't really get chalk pencils to write or sharpen very well, but the white charcoal pencil is my favorite marking tool.

I’ve never had success with chalk, despite trying several different types. I either couldn’t see the marks when I needed them, or they stubbornly stayed there when I did not.

These days, I use crayola washable markers. As long as the finished product is washable, I’ve never had a problem getting the lines out, even if they’ve been there for some time.

With dark fabric, I use slivers of regular bar soap. I’ve a tin where I save them. Cheap, it brushes off dry, washable, effective. I’ve been doing this for years. I’ve tried chalk, Bohin pencils, other methods, but prefer the plain old soap.