First you need to remove the old wax from the base. To do this I filled a pot with boiling water- about 3cm deep. You then sit the yankee candle inside to allow the wax to melt and it should come out whole. To get rid of any excess bits of wax you can pour some boiling water inside the jar, swirl it around and quickly pour out. It's probably best not to pour the waxy water down your drain ! It doesn't matter if it's not perfectly clean of wax as you will be filling it with soil anyway.
It doesn't have to be a Yankee candle holder you use. I also turned this silver antiqued Anthropologie candle holder into a planter as it was too beautiful to throw away!
Here are some more ideas for recylced planters. All these images can be found on my Pinterest page here.