When we talk about using a double boiler to make a product, some of us are using a double boiler (a pot on the bottom with some kind of insert in the top), but most of us make our own by putting heatproof containers - like Pyrex - into a pot of very hot water and heating our ingredients that way. The whole point is not to subject our ingredients to direct heat, so we use the water to heat the ingredients. So technically, it's not a double boiler - it's a pot of water into which we put our heatproof containers. It's just easier to call it a double boiler because people get the idea of not putting the product directly into a pot and putting that on the stove.
I actually use an electric fondue pot as my double boiler, so there's no top and bottom to it as there would be with a double boiler. It's just an open pot with a heating function I can control, as you can see from the picture.
If you are using something like a pot with very hot to boiling water or a fondue pot, a little metal ring to keep your glass ware from touching the bottom of the container is a good idea (it keeps them from breaking prematurely). You can get these at dollar stores for $1.25 or so, and they're so worth it!
If you're new to lotion making, check out the basic lotion making instructions here!