Here is what I use to make wigs.
I use a two part epoxy and a piece of thin fabric to make the wig cap.
The saran wrap is held tight to a wig mold with a rubber band. I made my own wig mold from polymer clay. I use the mold instead of the doll's head so I don't ruin the doll with misplaced glue and rough handling. Also The head mold is great for boil perming to style the finished wig.
Cover the saran wrap with sheer stocking fabric. Pull tight over the head mold and secure with another rubber band.
Mix the two part epoxy and spread the glue thinly over the fabric up to the hairline.
Make sure the hair line goes down to the nape of the neck for a secure wig.
Let the epoxy dry over night or at least several hours. Trim excess fabric away.
Now your wig cap is ready for applied hair. Make sure it fits the doll and clears the ears. You may have to trim a bit of the epoxy. It's hard and brittle so use small plier snips to trim.
The saran hair is glued to the cap using Fabri-tac glue. It's a matter of personal preference how to do that. Trial and error are your best teachers. I look at other people's work to get clues and try to work out a method. You can make a weft and glue that to the cap. You can glue little bits of hair to the cap. A combination of methods can be used.
For styling it's the same as a rooted doll although you can prep parts of the wig before application. You can boil perm the wig into a permanent style after the hair is added to the cap. Or you can plaster the hair down with a water soluble varnish--I have not tried this that but it appears that's what other's do.