You can opt to purchase LEDs if you wish. You'll want LEDs that are rated at 20,000mcd (millicandela). That's bright enough to see them in sunlight. If you buy LEDs or have some on hand, you can skip straight ahead to step 7.
This, up to step 6, are all about extracting an LED from an individual Christmas tree bulb.
First cut an LED from the string of lights, strip the ends of the wire with a #14AWG (American Wire Gauge) wire-stripper and test the LED. Do this for each LED. You don't want to bother breaking into a light only to find out later it was burnt out. LEDs last a long time, but they can burn out.
I like to fiddle with electronic circuits from time to time, so long ago I built myself a dual-voltage, dual-polarity power supply and incorporated some other features into it, such as some fixed rate frequencies, a variable-rate frequency generator, and an LED tester. That's what you're looking at in the image.
The LED tester is simply nothing more than a 1KΩ (kilo-ohm) resistor connected to the +5-volt portion of the power supply. The LED is then connected to the other end of the resistor resistor and the ground of the power supply.
You can make your own LED tester from a 9V battery and a 1KΩ resistor. Tape one lead of the resistor to the positive (+) terminal of the battery. Then briefly touch one lead of the LED to the other lead of the resistor and touch the second lead of the LED to the ground (-) terminal of the battery.
LEDs light in only one direction. If it doesn't light, swap the leads around and try again. If it still doesn't light, toss it out and try another LED.