This advertisement is providing us information about the fourth and fifth methods of making a gown. Use a pair of Stays or an existing Gown for a pattern.
"Dresses equally cheap, plain Gowns from 2s. to 4s. Sending a Gown or Stays that fit prevents Ladies the Trouble of trying on their Cloaths."
We would call this today pulling a pattern. We do this when we study extant garments. That is how all those patterns of extant clothing get into books. It is not a difficult skill to learn.
If a gown fits, why would a professional dressmaker not be able recreate the fit? If the stays fit, the gown will fit. So two more ways of making a gown added to the list.
2. Taking the measure.
3. Make a pattern from Stays that fit.
4. Make a pattern from a Gown that fits.
5. In person.
Keep in mind that the gown style is changing very little from year to year until 1775ish and you can better understand how these techniques can be easily applied. You are not creating a completely new silhouette, but rather just recreating the old one in new fabric. It makes sense, especially for those not within fitting distance of a Mantua Maker.