Step 1: Preliminary Direction: Study the LED
Study the LED. The LED is bipolar, with a positive (+ or anodal) prong and a negative (- or cathodal) prong. Usually the positive prong is longer.
Step 2: Preliminary Direction: Study the Battery
Study the battery. The positive side is marked with a "+" sign, while the negative side is usually unmarked.
Step 3: Preliminary Direction: Determine LED Polarity
Determine the polarity of your LED by holding the battery between the LED’s two prongs. This project mostly consists of sewing conductive thread to connect the positive sides of the battery and the LEDS, and then sewing to connect the negative sides of the battery and the LEDs. (See pictures below: the purple line is the Positive Thread connection, and the green line is the Negative Thread connection.)
Note: I didn't have time to take a new pic, so the picture below is from the LED Throwies Instructable. Thanks, Q-Branch!
Step 4: Cut the Hearts
Cut three felt hearts (two are shown below):
2 same-sized hearts, about 3”x5”
1 smaller heart to hold the battery, about 2”x1”
Step 5: Set up "Positive Thread"
Thread about 1.5’ of conductive thread from a needle. In the center of one larger heart, sew at least 20 loops to make a nice thick knot, which will be the connection to the battery’s “+” side. A long length of thread should be left hanging—you will connect all the LEDs’ “+” sides with this thread.
Step 6: Set up "Negative Thread"
Thread about 1.5’ of conductive thread from a second needle. In the center of the smaller heart, sew at least 20 loops to make a nice thick knot, which will be the connection to the battery’s “-” side. A long length of thread should be left hanging—you will connect all the LEDs’ “-” sides with this thread.
Step 7: Stitch the Battery Heart
Stitch the battery heart to the pillow front with the Negative Thread.
Step 8: Set up first LED
Poke an LED into the top of the pillow front. (If it's hard to poke the LED through, use a needle to pre-poke holes). The LED's positive prong should be closer to the pillow front's edge.
Step 9: Connect first LED
Stitch the Positive Thread and Negative Thread close to the LED. Tie the Positive Thread to the LED's positive side. Tie the Negative Thread to the LED's negative side.
Step 10: Test first LED
Now let's test the light. Lay the battery, positive side down, on the pillow front's center knot.
Step 11: Light first LED!
When you press the battery heart to the battery, the LED should light! If it does not, make sure that none of the positive connections (thread and LED prongs) are touching any of the negative connections. (When positive and negative touch, this is a "short circuit" that will not work.)
Step 12: Lay out remaining LEDs
Poke the rest of your LEDs though the pillow front in a design that looks nice to you. If you're short on time, use fewer LEDs.
Step 13: Note: messy back
The back of the pillow front will now be a bit messy. You'll need to be careful with this when you connect and test each light-- always make sure that the positive and negative connections do not touch!
Step 14: Connect remaining LEDs
Repeat steps 9-11 for each LED. If you test an LED and it doesn't work, check your positive and negative connections. If the LED still doesn't work, replace the LED.
Step 15: Close Positive Thread loop
When all lights are connected and working, stitch the Positive Thread to the pillow front's center knot. The Positive Thread should now form a closed loop. I've drawn a purple line below over the path that the Positive Thread should follow. Note that some of this thread will be sewn behind the smaller battery heart.
Step 16: Close Negative Thread loop
Stitch battery heart to the pillow front using the Negative Thread, and then stitch the Negative Thread to the battery heart's center knot. The Negative Thread should now form a closed loop. I've drawn a purple line below over the path that the Negative Thread should follow. Note that some of this thread will be sewn behind the smaller battery heart. Warning: making this loop may be a little tricky!
Step 17: Glue LEDs in place
Finish off the pillow front by gluing the LED prongs in place. The glue not only secures the LEDs, it also insulates their prongs.