Pantalets were made of bleached white cotton. I have a least 1 pair in my “trunk”. I need to dig them out. The pattern is simple and easy to figure out from looking at a pair. Trimmings of tucks, lace, or embroidery on the ones I’ve seen were similar to cotton petticoats.
This pair is from the Met Museum.
The pattern below is from La Mode Illustree: journal de la famille magazine 1863 page 93 Google Books. I’m curious about how it works. I think the vertical center line is a cut line to get the 2 legs of the pantalets. I think the far right and left vertical edges are the lower hem of the legs. I’ll have to try cutting out a doll size one to see for sure.