Part 1: Preparations and materials
Please check out the 18th century fabrics page.
The Française was not exclusively upper class, but the sheer amount of fabric that goes into its construction made it unaffordable for anyone who didn't make a comfortable living. For the middling sort, it served as sunday best. This means that the fabric should be of the better kind - either silk or printed cotton.
The most suitable silks are taffeta and satin (i.e. duchesse, definitely not the slinky kind), brocade, damask and moiré. The latter three are hard to find and quite expensive. Do not use dupioni silk: It may be the chapest kind, but anything slubby would not have been used for a française. (Incidentally, I frequently find that Dupioni from a shop around the corner costs more than taffeta ordered directly from India via the internet.) Finding a printed cotton with a suitable pattern is extremely difficult, especially for a novice with an untrained eye, and suitable cottons can easily be more expensive than a solid-colour silk taffeta. If you're really on a budget, some solid-colour artificial silk taffetas can look quite convincing, especially if they're rayon or acetate rather than poly-something.
Please consider carefully before buying artificial anything. Making a française is a lot of work. Work usually is much more expensive than fabric. If you're a student, you'll probably have more time than money, so choosing a cheap fabric makes sense. If you earn enough money to afford a bit of luxury now and then, your time is too valuable to choose an artificial fibre just because you save a measly 10%. On the other hand, if it's your first try at historical sewing, you may not want to spend too much money on something that you may not be perfectly happy with a year later. In that case, use a sewing machine and save your hand-sewing for later, when you feel more confident - and save up for a proper silk meanwhile.
I've been asked about the fabric weight. Not an easy question to answer because the "drape" or "hand", as fabric fanciers put it, is more important, i.e. the flowyness or stiffness of the fabric. Just to give you an idea, silks up to 70-80g/m² are too light, heavier silk up to about 180g/m² should be OK. Sometimes silk weights are given in mm (momme) - in that case, go for something upwards of 20. My heaviest française, made of cotton, weighs 2.5 kg, lining, skirt and all. The weight is not a problem, but the fabric is a bit on the stiff side. Consider that heavier fabrics weigh down skirt supports, especially wide ones, so that you may need stiffer (and heavier!) steels in the panier.
As for the amount, for those who want it in a nutshell: If you economically puzzle the pattern used here onto your fabric and aren't taller than 165 cm, 7-8 metres @ 150 cm width or 10-12 metres @ 90 cm width will suffice, a small safety allowance included. 1-2 metres more for patterned fabrics.
If you're taller, wider, want to use a different pattern or want to know exactly:
Stout figures need only insignifically more fabric, tall ones need significantly more. The calculation for a relatively economical pattern (i.e. small panier) and 150 cm fabric width goes roughly like this:
*) That's the full height head to toe. Makes for a small train.
If you're above 175 cm tall, add 10-15 cm to keep the train in proportion. No
train is not an option!
**) Makes for 3 m skirt circumference. Don't go for less. If you're very stout, add at least half a metre.
You may think that this is an awful lot of fabric. Yes, it is. You may think that you won't need as much because you're petite. No, believe me, you will need it. It doesn't make much difference whether you're slim or fat. Even if you're on a budget, an 18th century robe is nothing to be niggard about. At a fabric consumption of 7 metres or above, finding out too late that you're half a metre short is reason enough to bite your own leg off - especially if you're on a budget! So don't let the fabric mature on a shelf, but get to at least the robe-cutting stage while there is still something left of that bolt in the shop - or calculate generoulsy from the start.
If you're on a budget,
If you're very tall, consider that you will have to pleat more fabric into the back (i.e. wider back parts), otherwise the pleats won't fall gracefully all the way down. The back parts should also be a bit longer.
If you plan to trim the dress with ruchings &c, add one to two metres to the above. There's also a fabric-saving version of the petticoat (cheat petticoat) that was not uncommon and showed up even in the inventory of Mme Pompadour1: The petticoat was made of plain linen and only covered with the fashion fabric where it showed, i.e. in front and partly up the sides and back. Piecing the fabric together like that means more work, but in a time when labour was a lot cheaper than fabric, the additional cost was negligible.
For the lining, you need roughly 150 x 75 cm of a firm linen-weave fabric. The lining will hold the whole construction up, so it mustn't be too weak or be distorted easily. Also good to have: an undefined amount of cheap fabric for mock-ups.
Something like a dressmaker's dummy or preferably a patient friend. Since modern commercial dummies don't have the right (corseted) shape, a live helper will still be important; the dummy can at best replace the helper in between the critical steps. If you only have a modern, but adjustable dummy, set it to 1-2 sizes smaller than you are, put your stays on it and stuff certain places with rags.
2-3 m waistband tape for the petticoat, one or two metres of narrower (0.5 cm) tape for the back lacing, 50 cm of 1.5-2 cm wide tape for the stomacher. 2.5 metres of light boning. Some fabric to protect the underside from rubbing and dirt - inexpensive, not much heavier than the top fabric, and smooth. It should be the colour of unbleached cotton or linen or match the top fabric or go well with the top fabric while being inconspicuous. Reckon about 80 cm @ 150 wide or 120 @ 90.
The usual suspects: Sewing thread, basting thread, needles, scissors etc. If you're using a silk fabric, go for silk sewing thread. Otherwise, linen thread. Polyester thread is unauthentic, of course - and cotton as well. It was not until a bit after 1800 that cotton could be spun strong enough for sewing thread.
For trimmings of the 1750s/60s, pinking shears or a proper pinking tool are very important. Depending on the era and very optional (i.e. if in doubt, leave them out): fine linen or cotton bobbin lace, metal lace, satin or taffeta tape, artificial flowers. Contrary to the cliché, embroidery, spangles, feathes and tapes were not used as decoration on the dress; lace, flowers and feathers were rare. Mostly it was pinked strips of fabric, ruched or laid into pleats, fly fringe and sometimes chenille. Look at paintings and photos of extant dreses, do!
The most important preparation is to determine which period your dress is to be. "18th century" is just too vague. A française wouldn't be appropreiate before 1710 or after 1780. If you can't decide or don't care, follow this tutorial to make a robe of about 1750. But maybe you have a prefgerence for a certain feature: Waisted or loose, plain or with lots of trim, big or small skirt? If so, make sure that all those features occurred at the same time. Some of them simply don't go together. It wouldn't be worth all that work only to end up with an incongruent mix of 1740s boxy panier, 1730s sleeve cuffs and a 1760s waist seam.
Most books recommend washing the fabric before cutting in case it shrinks. In case of silk, however, that is risky: Silk tends to lose some of its shine and stiffness. Taffeta in particular tends to retain crease lines from spin-drying even if you iron it for hours. On the other hand, washing will become necessary sooner or later, no matter how careful you are. Washing only the dirty parts won't do: You'll have to immerse it all or risk water stains - or a part shiny and stiff, part limp garment. In case of silk taffeta, damask and brocade, I therefore suggest that you don't pre-wash and put off cleaning as long as possible. All the more reason to attach aforementioned cheap hem protection fabric to the inside of the train.
Next step: the pattern
1) Exhibition catalogue "L'Art et l'amour", Hypo-Kulturstiftung, München 2002