Im going to be trying to make a more complex hair piece so I would also be grateful for any references or tips.
you can sew it to the cap. Depending on what sort of cap you're using. Glue works but it makes a bit of a mess and it's more tricky to get a neat parting because it leaves well.. shiny hard residue on the ends which is visible if you're not super careful.
If you get a thread that's a similar color and use small stitches then it shouldn't be visible under the yarn.
You could do a partial yarn reroot and glue on the rest.
If you have a sewing machine you can make 'wefts' for the wig, then sew said wefts onto the wig cap. If you do use a glue instead I recommend finding something that dries translucent to clear, and to protect your doll while gluing so that the glue doesn't get onto the doll itself and make any kind of unwanted texture/color.
You can make wefts with glue, too, it just needs to be a good quality glue that won't dissolve if you expose it to water. ( some glue is very sensitive and even humidity can cause issues.)
I personally just make wefts with glue, though I want to try out making wefts via the sewing machine method. With glue it's easy because you can trim the glued part and also cut the wefts into smaller pieces.
Are you using the yarn whole or processing it into fiber? I process it like one would wool. My yarn is four strand acrylic, it processes/cleans up very nicely and when ironed carefully it becomes very flat and satiny.
Its been a few months, Took some time to experiment with some alpaca yarn in a dirty blond color. Was gifted to me by a very crafty mother of a friend.
I made 3 hair pieces varying the application method.
I made my first wafts and they turned out very good for what I was looking for, as a note - modge podge makes the roots a little stiff for my taste so I might experiment with different adhesives to see about results but that will be for a project in the future.
I found that winding the wafts into a spiral and then applying that to the crown and then blending in some smaller wafts around the edge created a very cohesive piece.
I havn't brushed them out since they've dried completely but I plan to take pictures when my current project is finished.