Cut off the sleeves or make a sausage from another part. I used an overlock type of stitch on my sewing machine. Zigzag plus straight stitching should hold it, but has a tendancy to stretch the seam. Give it a slightly smaller end. This will be the head. Don't forget to leave an opening to stuff. If you are clever you could unravel some wool and use it to invisibly sew up the hole, through the loops of the knitting.
You'll now need some felt*, a darning needle and assorted embroidery silks.
The design is up to you. It doesn't have to be symmetrical.
I used the pullover's cuff for one of the beanies and hand-knitted the other with No. 2.5 needles and a scrap of very fine wool I had left over.
* you could use regular fabrics and hem them all around. It would make a completely washable doll, but is more work than felt.