I've looked for a nice site that tells how to make trim for 18th century garments, but the closest thing to helpful that I've found are sites on modern ribbon crafts or quilt applique.
The first trim I want to show is Ruching.
This is a ribbon or fabric strip that is gathered and stitched down.
The most common place this was seen in the 18th century was on Sack-back gowns or Brunswicks like this one.
This is Lady Mary Fox, and I want this Brunswick... without the dog.
If you are using self-fabric, (the same fabric the dress is mad out of), which was the most common thing to do in the 18th century, you have to decide if you are cutting on the bias or the straight grain. Each looks different, so try it out and see what you like. (If you are doing lots of curves, cut on the bias). This takes up quite a bit of fabric, so plan on buying at least 2 yds for trim.
Sew all your strips together end to end... make sure all your seams are on the same side.
Next you need to finish the edges. Sometimes it was turned over, pinked, or a decorative stitch was added. I've seen some paintings where it looks like there is separate trim of the same color. This could be dyed lace, or a crocheted edge... I'm not sure.
There are many variations in the stitch, here are 2 basics.
The first is a straight running stitch that is gathered. Make sure to gather and then backstitch every 10 stitches if you are using one thread.
You can do 2 running stitches side-by-side and gather the material all at once.
Modern sewing machines do a great job if you are making doll clothes, costumes or are in a rush.
To make even gathers, measure every 3" and gather into 1", or something like that.
There is no substitute for hand stitching historic garments.
The next looks best on finished edges, wide trim 3" or more, or ribbon. It doesn't work well with pinked edges... which I'm using anyway.
Mark your trim at even intervals. Sew a running stitch from point to point, looping over the edges. Pull the thread straight and you get a squiggly looking ruche.
A nice variation is a curved line rather than a triangle, it's very historic.
These are the 2 finished ruching styles. They can be edged, stitched down once or several times, paired with beads, lace, other trims, etc. Just look at some paintings or historic garments.
The fabric I used here was cut on the bias and is a very narrow scrap, about 3/4". Use wider stuff unless you are making doll clothes or sleeve trim.
A nice combination is stitched pleats with ruched edges. This was done by sewing gathering stitches on the straight fabric, stitching down the pleated center and then pulling the threads tight on the outside. You can see this in several sack back gowns from the mid 1700's.
Here is a doll dress I made as a test garment for a sack-back gown from one of Diderot's illustrations. This was more to figure out the lining than anything else, but it's a nice doll dress... to bad I still have to make the doll to go with it. The ruching isn't perfect, but it's a nice example.
I will eventually make a sack-back gown for myself and it will have loads of ruching on the gown and the petticoat. First I need paniers and fabric... then I need some serious time.
Why, oh why do I want a dress that will make my hips look even bigger???