The dress form cover pattern I have on Craftsy can also be stuffed to make a dress form.
Use firm non-stretch fabric to make the cover. Sew the pattern as in the instructions, but with one modification: Instead of leaving one side open, sew both sides closed.
If you want to keep the neck and armhole from bowing out, you could cut pieces of cardboard to fit the neck top and armhole areas and glue them on the inside with fabric glue. I didn’t do that in the pictures below, and you can see the armholes pushed out a bit.
To stuff the dress form, I rolled up some denim until I had a firm cylinder that would fit in the neck, then stuffed the rest with a combination of fabric scraps and polyester fiber fill. When I had stuffed it about halfway full, I inserted a vase, then stuffed around that. At the bottom I just pulled the drawstring tight. You could experiment with putting an oval of cardboard, wood, or foam in the bottom to give it a better shape. I stuffed the one below quickly – with a bit more effort, you could make one that is not as lumpy as mine.