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.