The following instructable details the process of building a lighted fabric jellyfish. While we created these as a part of an automata aquarium, it may be useful in your other projects as well.
These jellyfish were controlled by a small arduiono using the example blinking method found on the arduino examples website.

Supplies:
-Slightly Sheer Fabric
about 1/4 yard
-Thread to Match the Fabric
-Quilters Batting (any kind)
-Ribbon
-Pony Beads
-Washers
-Electrical Wire
-Several LEDs
-Power Source
-Arduiono (optional)

Tools:
-Sewing Machine (or needle to hand sew)
-Pinking Shears
-Scissors
-Fabric Chalk

Step 1: Cutting

You will have to cut out three different circles, two out of the fabric and one from the quilters batting. All of the measurements can be adjusted as long as the size ratio is kept.

Use the pinking shears to cut all of the circles, especially the fabric to prevent fraying.

Cut the first circle from the fabric with a 5 inch diameter. The next two circles, one from fabric and the other from batting with a 4 inch diameter.

Step 2: Pinning

Align the three circles into a sandwich with the batting in the center, the large circle on bottom, and the small fabric circle on top.

This is a bit tricky, fold the edges of the first circle under, like making a hem, consider this to be the new edge of the large circle. Now fold the edge of the outer circle over the smaller circles, making an enclosed sandwich. Pin the overlapping edge folds in the second step so that they will stay in place while sewing.

Now there is one more thing to pin. Take eight pieces of ribbon and place them equally along the jellyfish edges. Pin the ribbon tentacles in place without turning the circles over.

Step 3: Construct the Soft Body Elements

Sew the edges and tentacles in place with either a simple straight stitch or, even better a tight zig-zag stitch.

Now sew a gathering stitch. A gathering stitch consists of two rows of very loose straight stitches sewn in parallel and close to each other.

Gather the jellyfish together to give it that characteristic shape. Gather both gathering stitches evenly and make sure to pull gently when gathering. If you pull too hard on the threads, they are very likely to break.

To get a nice even gather, put the body over a container, I used a juice bottle.

Place an eyelet in the center of the jellyfish body to string wires or a method to hang the fish.

The final step, which is optional, in the construction of the soft body elements is to thread some pony beads onto the tentacles. Three beads provide a nice spacing. Seal the last bead onto the tentacle with a bit of glue to keep the beads from falling all the way off.

Step 4: LEDs and Weights

We used several LEDs per jellyfish in order to have the shine brightly through the three layers of material and be visible in a fair amount of light. Each jellyfish had 5-6 LEDs.

Twist the corresponding LED legs together and solder a piece of wire to each collection of legs. This will create a circuit that can turn on all of the LEDs with a command intended for one.

Now thread the wires through the center eyelet with the LEDs ending up inside the jellyfish.

Connect the jellyfish to a power source and watch them glow.

The jellyfish can be made self sufficient by using the same technique in the traditional throwie and replacing the tied in LEDs.

If this is going to be used in something like a shamelessly plugged automata aquarium project, some weights might also need to be added. We used a little tape.