I ordered a candle from an ETSY seller a few months back. This seller uses organic, soy wax and scents the candles with essential oils. LOVE IT! The candle I purchased was in a funky jar (really cool looking, but not so practical for a candle). I burned it down and there was a TON of wax left on the sides. I didn't want to waste the wax, so I picked it out with a knife (good thing about soy wax, it's super soft). I melted the wax in a sauce pan on low heat.
I had purchased some cotton wick and reused a wick holder from a tea light. What I did was pinch the wick holder with a pair of plyers. This releases the old wick and then I thread the new wick in and pinch it so that the new wick holds in place. Sometimes I have to use a needle or pin to push the old wick out.
I wound the extra wick onto a pencil and made sure the wick holder was in the middle of the glass container. I poured the warm/liquid wax into the glass jar carefully. After the wax cooled, I trimmed the wick.