Step 1: Getting your supplies...
1. Some 10mm Non-diffused or diffused (your preference) LEDs
2. Some CR2032 3V Lithium Batteries 2 per LED used
3. Some clear tape (or whatever tape you like)
4. A tree to hang them in.
5. A party of giggly 8 year old girls (optional)
First, make yourself some "Throwies". Detailed instructions can be found at http://www.instructables.com/id/LED-Throwies/
Otherwise, get some LED bulbs. I used 10mm non-diffused but if I had it to do again, I would use diffused as these were pretty much directional. Almost like a mini spotlight.
Then you'll need a power source. For this I used CR2032 3V Lithium Batteries. I used 2 for each light.
When you assemble the "Throwies", make sure and pay close attention to the leads on the LEDs. One is just a bit longer than the other. This is the positive side (+) which goes to the positive side of the batteries.