Measure body of your pullip around the hips and the length of the torso. Cut a square a little bigger than this (for the seam and optional hem)
Then cut a square that will fit around the shoulders and fold two or there times to cover the upper arms.

Fold each square in half and sew them up the open side so they make a loop.
Sew the bigger loop at the opposite side of the seam as shown in the pic, but just about a quarter of the shorter length.

Roll the neck over the top of the body. The arms go through the neck. You could tack the neck at the back as well, but I didn't.