Women's fashion in the 1700s was mainly characterized by the overlarge skirts people now associate with the Rococo period, which had reached an incredible width by the middle of the century, often sticking out farther than a woman's outstretched arms. Dresses of the day were designed around this feature, developing later in the century into the smaller half-panier skirts that modern people are more familiar with, and then again into wide hoop skirts. The wide skirt feature is the main draw for costumers, and though complex, is not impossible to create.
Create a chemise out of white linen fabric, either by using a pattern (see Resources) or drafting your own pattern from rectangular sections of fabric. Divide one large rectangle down the center width, and stitch two small rectangular sections to the sides, matching the centers. The longer rectangle, folded, should reach your knee, and the small rectangles should reach your elbows. Also stitch two triangular gores to the outside ends of the large rectangle, so that the chemise fans out. Cut a neck hole at the center of the large rectangle. Stitch the side seams of the chemise and turn under the hems to complete. This is the bottom layer of your 1700s dress. You can cut the neckline later to match your bodice.
Create your own stays, or corset, pattern using the custom corset pattern generator (see Resources). Cut two copies of the pattern pieces in four types of fabric: coutil, coutil again, cotton lining and your chosen outer fabric. Baste the outer fabric to one set of coutil pieces, and the lining to another set of coutil pieces. Stitch the back outer pieces to the back lining pieces along the center back and the front outer pieces to the front lining pieces along the center front, leaving openings for the busk prongs. Insert the busk pieces into each front side and stitch beside them, poking holes in the fabric for the knob side. On the back pieces, mark the grommet placement along the center back, leaving room for boning on each side. Insert the grommets. Sew the other panels together on the outer and lining sides, then sew the lining to the outer fabric. Stitch the vertical boning channels on all panels and insert the boning, then bind the top and bottom edges. Run lacing through the grommets.
Make a hoop skirt to wear over your corset. Sew the skirt like you would a knee-length petticoat, by cutting two large rectangles and stitching the sides together. Stitch a length of twill tape around the hem to create a boning channel, then stitch another boning channel 4 inches up from that. Continue adding boning channels until you reach hip level. Gather the top of the skirt and stitch it to another length of twill tape to use as a tied waistband. Insert hoop boning into the channels, then stitch horizontal pieces across the inside cavity of the skirt so that it holds your chosen shape.
Make a petticoat out of your gown fabric. This petticoat is a simple skirt sewn from rectangular sections that is large enough to cover your hoop skirt. Leave the side seams of the skirt open 5 inches at the waist, and stitch two separate waistbands to the front and back of the skirt that are tied at the sides. These openings can also function as pocket slits, if you wear hanging pockets under your hoop skirt.
Make a stomacher that is slightly taller than the front of your corset. You can use the first two panels of your corset pattern to make the stomacher, and construct it in much the same way, including the boning. The stomacher is worn pinned directly to the corset front.
Create the gown itself, often referred to as a robe. Use a robe polonaise, anglaise or francaise pattern that you like, or draft your own pattern using a fitted non-stretch T-shirt. Put the T-shirt on your body, with the sleeves and collar cut off, and have a friend help you draw on it. Draw a curved concave line from the top of the neck edge at the shoulder seam down over the center of the bust, then a convex line from that point down to the waist. Draw all around your body at the waist point, and draw a line on each side from the underarm to the waist. Take the T-shirt off and cut along your drawn lines, then take apart the shoulder seams. Use these for the bodice pattern pieces.
Pin your T-shirt pieces to your gown fabric and add 1/2 inch seam allowances. For the sleeves, lay the T-shirt sleeves along your fabric and extend the lines to a 3/4 sleeve length, then add seam allowances. Cut the pieces in gown fabric and lining fabric. Stitch the bodice together like a T-shirt, then cut a length of gown fabric equal to the back side of your petticoat. Stitch this length to the bodice bottom in the back, stopping at the front edges. This gown is worn like a coat over the undergarments.