With one hand, hold the candle container/jar with a towel or hot pad. With the other hand, take 3-4 paper towels and wipe out all of the wax, soot, burned wick pieces etc. I'll generally use several paper towels for each jar. Be sure to wipe out both the inside and the outside...as well as the bottom of the candle container.
While you're wiping, be sure to inspect the candle container thoroughly for any chips, cracks or other imperfections. Sometimes little hairline cracks will become apparent when the jar is hot and you're wiping the wax out of it. If you find a crack or imperfection, throw it out!