search google for pictures of "skeleton tights" for a guideline and using whatever you have to draw on the fabric (chalk, pencil, etc), mark out the bones on one of the legs of the tights.

start painting in the bones on the leg of the tights with the white fabric paint.

before it dries, fold the leg over so it is face down on the other leg of the tights, this will transfer some paint to the other leg which will give you a guideline on where to paint on the other
leg. this will mean that the two sides will be symmetrical.

you will need to use a good few coats of paint to make the white opaque. leave to dry and you now have skeleton tights :)