How would I design a free standing cardboard sign that lights up?

I want the sign to light up so it can be seen at night but not to require an electrical outlet, making it suitable for outdoor use. I've seen it done with what looked like clear plastic cords with lights strung through them but I have no idea how it worked. Does anyone have any ideas?

Okay, fair enough. A laminated cardboard sign, maybe.

They make small lights that are compact and have a battery pack with a switch to turn on and off. I've found them at christmas time, but I bet Home Depot or Lowe's carries them year round. You would have to cut a hole in the sign for the light to fit through, then you could turn the lights on and off from the back. Or, just look for anything similar that is battery powered.

You would need some form of power. My thoughts think that battery would be the most effective, but perhaps solar could be considered also. A car battery is a very effective, but short term form of power, and it can be recharged effectively. Solar could work, but in order to work at night, you'd need to store the energy, the only long term method would be to recharging batteries, and that just isn't efficient...yet.

Once you have a power supply, you then can run your cardboard sign that lights up.

Cardboard is a real poor choice for outdoors.
Those "clear plastic cords..." were low voltage lighting,
and need an outlet somewhere near.
There are fairly cheap solar powered lights that you
could modify to shine on your sign.