There are many different brands of "thinners" out there. A Google search for cannabis concentrate thinners gives a few results.
They come unflavored, flavored, with extra terpenes added, you name it.
Then you heat up your concentrate just enough so it's liquid, and mix your thinner in.
Once it's thin enough, use a large gauge syringe and put the vape juice into an empty cart.
Not sure if Ebay or Amazon, but I got a kit that had syringe, mixing tool, and 5 empty carts.