Step 7: The Petticoat
The general make-up of skirts has been moved to an extra page. If you don't mean to attach trimmings, your robe is finished as soon as you've completed those instructions. See here to learn how to put it on.
Most skirt trimmings run horizontally, so it makes sense to get the hang of the skirt right before attaching the trim. So make up the petticoat according to the instructions on that extra page first.
It is only from about 1750 until the 1770s that skirts are trimmed at all. In most cases, the trimmings consist of strips of dress fabric that have been pleated or ruched or made into puffs. If the dress fabric is taffeta or satin, the edges of the strips are usually pinked. Other fabrics are usually rolled or very narrowly turned.
As for what kind of trimmings are to be applied in which way, that's a matter of taste. I recommend studying paintings and/or pictures of extant garments, e.g. in the books listed here. Typically, there's one wide (25-30 cm) horizonal strip, at least 100 cm long (after pleating or ruching) that sits just above the hem and runs parallel to it, and maybe a narrower (5-10 cm) strip waving up and down just above. Another narrow strip (4-6 cm) runs along up the front edge of the gown, around the back and down again.
Next step: Variations