I like to make a toile for a garment if it is the first time I have made something for that person. A "toile" is a trial garment, used to test out the pattern for a first fitting. In some places, people would refer to it as a muslin.
You can make a toile out of any fabric I suppose, but I generally use an unbleached cotton or muslin, but not a wimpy drapey one. This one is a bit more tightly woven- more like ticking in feel. You don't want to spend a lot of time or money on the toile fabric because it is going into the round file after the fitting. It is important though, to chose a fabric that will give you a good "reading" of your pattern, and isn't too far off the weight and drape of the final fabric. For instance if you are making an overcoat, you could choose some heavier cheap "unknown fibre" yardage rather than a thin muslin that would be more suitable for a blouse.
I like a fabric that is easy to draw on as well- either with pencil, tailor's chalk or a sharpie marker.
What you want to avoid is making changes to the pattern that an unsuitable toile fabric may cause you to do.Once you have a toile fabric, give it a good steam press or even wash it so that you aren't accidentally shrinking your toile as you are making it. Unwashed muslin will easily shrink at least 1/4" t0 3/8" in length on a waistcoat if you don't press it beforehand.
In a toile, it is important that you leave enough "inlay" or seam allowance to make alterations in the fitting, or in some cases, like trousers, to sew a change without having to recut a brand new toile. This means giving a bit of thought beforehand to the possible fitting issues and the changes you might need to make as you fit.
You have to leave enough allowance to do this without distorting the fit.
On a waistcoat I leave inlay at the shoulders and sides that are pressed in one direction rather than pressed open. This makes it easier to unpick and repin without fighting the pressed back seam allowance. I leave extra to turn back at the CF to give some stability- same as the hem- it will give a better idea of the finished line. I don't bother to turn the armhole seam allowances back, but stitch the line and cut about 1 cm away through the curve and leave a bit more up at the shoulder area.
You need to see your design lines from the right side of the garment, so use whatever method works for you -tracing or machining or hand basting the lines as needed. We usually try to machine the lines to the right side as it saves time. Mark your details like pocket or button placements and sew your toile up with a long machine stitch that won't be too tedious to unpick, or baste by hand securely if you want to. We always hand baste the fronts back and the hems up as it is really irritating to try to unpick those in a fitting if they are done by machine.
Give it a good press as you go- make it look good.
It is also important to include the elements that will affect the fit in your final garment, put shoulder pads in and crease the trousers for instance. I don't think you should fully canvas your toile or make fly buttons by hand and you don't have to put the sleeves in a jacket at this point but try to get close to what you want it to look like in the end.
In terms of saving time, fabric and money, a toile is well worth it. You may not think so if your toile requires little, if any changes, but you will appreciate it on those occasions when things don't go as smoothly.
It was made more clear to me recently as I was able to track the times of three similar jackets made by the same person this year. The jacket that had a toile made for the first fitting took about ten hours less than the jacket that was cut directly into the fashion fabric, fit, taken back to flat then finished.