The easiest way to clean your votive candle holder is to never let the wax adhere to the glass in the first place. And, here is how you do it.
Simply add about 1/2″ of water before inserting the candle into the holder and then use as normal. The water in the bottom prevents the wax from sticking to the glass. The votive candle will simply pop right out when it’s time to replace the candle.
There may be a little soot and minor wax but nothing a quick, soapy bath won’t fix.
If you already have wax on your glass, there are a bunch of videos that suggest the following methods for cleaning: boiling in water, microwaving, freezing. Some folks have cracked their glass votives by using these alternatives. So, please use caution.
Again, the best tip is never to let the wax adhere to the glass in the first place. You can always use refillable oil candles and never have to worry about a waxy mess again.