Summarize:

There are two main types of hair extensions: synthetic and real hair. Synthetic extensions are cheaper, and readily found in beauty supply stores and "punk" stores, such as Hot Topic or Clairs. Real hair extensions are more expensive, and can be found in salons and beauty-supply stores. Unlike synthetic extensions, they can be straightened and curled. Some synthetic hair extensions are heat safe. The packaging will usually indicate up to what temperature it is safe to curl or straighten the hair. Decide where you want the streak, then slide the handle of a rat-tail comb through your hair, and use it to create a part. You never want to add an extension to the top of your head, because the comb and weft will be visible. Clip your hair out of the way, if needed. Find the comb part, and turn it so that the hair is facing you. Hold the comb between your thumbs and forefingers. Push down on the middle of the comb with one of your thumbs. You'll feel it click as it opens. Turn the extension so that the comb part is facing your head. Slide the comb into your hair, right below the part you made. Make sure that the teeth are catching in your hair. Press down on the sides of the extension with your fingers. You'll feel it snap shut against your head. If the extension is too long, you can cut it so that it matches the length of your natural hair. If you used an extension made from real hair, you can curl it or straighten it to match your hair texture. Cut upwards into the strand to thin out the ends and make them lay like real hair. Use a lower heat setting on your straightener or curling iron. A high-heat setting can damage an extension, even if it's made from real hair. Your hair will now cover the clip from the extension, making it look more realistic. At the end of the day, be sure to take the extension out.
Choose a type of colored hair extension. Part your hair where you want the extension. Open the hair extension. Slide the comb into your hair, just below the part. Snap the comb shut. Adjust the extension, if needed. Flip your hair back to its natural part.