Hello. I would first soak the jars (if they are still in jars) in very hot water so the left over wax pops out. Then I like to make a double broiler with a pot filled with water and a bowl on top of that. Add your wax and stir it as it melts. You can also add color or scent at this time. Dip the bottom of the wick into the wax and put on the bottom if you glass. Then I like to take the top of the wick and wrap it around a pencil and lay that over the glass. It helps keep the wick in place until the wax sets. Last pour your wax in. Have fun!
Here is a tutorial https://www.hometalk.com/12504579/don-t-miss-this-what-to-do-with-left-over-candle-wax
Break up old wax into even sized pieces so it melts easier,melt the wax in an old Yankee candle jar(I know they can handle the high heat)in microwave(this way I don't have to use one of my microwaveable glass bowls & jar is easier to get wax out of too) or you can use a dbl boiler on stove top low heat place 1 smaller pan inside another larger pan with water in larger pan. Place a pencil/chopstick across top of heat safe jar or any heat safe item you want to use for mold using a clothespin attached to pencil/chopstick to hold wick in center of jar/mold then add melted wax slowly to mold/jar you may need to use metal funnel depending on size of opening. Here's how to make wick or you can buy them https://www.20muleteamlaundry.com/uses/arts-crafts/how-to-craft-candle-wicks-with-borax https://candles.lovetoknow.com/Homemade_Candle_Wicks
You can melt old candles or wax chunks in whatever glass container fits over an electric candle warmer.
You don’t. The best, and SAFEST way to make a candle is with a clean candle container, wax for container candles, wick chosen based on type of wax and the diameter of the container and, if desired, candle dye & fragrance oil. As a Chandler, I do NOT recommend ANY re-use of old wax if you have no idea which type was used to make it in the first place, especially since this is what dictates which wick is to be used. Making candles to be burned is a SCIENCE, and should always be done with safety in mind. Doing otherwise creates a FIRE HAZARD, so do it at your own risk!
Thanks so much, my fellow chandler! You actually made that a lot more clear than I did