Waterbed with tubes filling instructions?

Question:just received a waterbed that has tubes inside the pillow top. The tubes are "motionless" with a fiber strand that runs through. How do I fill the tubes, laying flat? Then how will I know if I've filled to the correct fill line? When we attempt to hold up, the fiber falls to the bottom. I'm assuming that they are not filled enough, since the bed sags in the middle.

Answers:
laying flat, fill until it overflows