1. Light the candle until most of the wax is in liquid form. With a tea light in this size, it usually takes about 15 minutes.
2. Once the tea light is in liquid form, add a few drops of the fragrance oil.
3. Take a metal chopstick or something metal to stir it with and carefully stir the wax so that the fragrance oil spreads through the wax evenly (it's ideal to do this while the candle is still lit to keep it in its liquid form and so that the wax doesn't stick to the chopstick as much, but I'm sure you can do it without too). Melt the wax off the chopstick so that you won't waste any wax.
4. Blow out the candle to let it dry, and you have yourself a nicely scented candle! Some of the wax will be gone, but you will still have enough to enjoy the nice scent.
Now, you can't really do the above method with a thick candle because it will take too long to melt, so tea lights or smaller candles work best. If you have a big candle that you aren't really using anymore, you can make new candles out of the wax. Melt the wax by using this method from mythirtyspot. She used scented candles, but if you were using unscented and wanted to make it scented, then adding and storing the fragrance oil in while it is melting in the pot would be the place to do it. Then follow the rest of her steps, and there you go! New candle!