I start by cutting a big handful of lengths of thick acrylic yarn. I knot them around a dowel, flat iron them, then start brushing them out with a slicker pet brush. You'll lose a lot of fibers, but keep it around for stuffing. Above is before and after brushing.
I cut the straightened yarn off the dowel, brush the blunt end to fan it out a bit, then lay it down on a sheet of silicone (which is on a board so I can move it around easier)
I brush watered down mod podge onto the blunt edge, making sure it's saturated, but not too thickly laid on.
After letting it dry for a bit, I peel the wefts off the silicone and flip them over to let them dry more completely.
After they're like, 99% dry, I trim excess off the glued area, and use hot glue to place the wefts on the head
For Mettaton here, I worked in circles around his hairline.
Where I'm pointing though, I stacked like 3 wefts to make sure his bangs over his eye looked fuller and had more volume
And here's the hair finished, pre-haircut! I didn't worry too much about making a nice "part" with this one, just kept circling around until it was full, and the black hides the finishing piece well enough.
Now to work on the second head!