How to use beef stock fat
At this point, you can do the lazy method of simply pouring the fat into a mason jar and re-hardening it in the refrigerator.
This is what is pictured above. Or, to make it completely free of any broth or bits, pour into a wide container such as an 8 by 8-inch pan. Let re-harden in the refrigerator, and then remove from the pan and turn upside down. You can then scrape off any bits or broth from the bottom of the fat.
Reheat to liquefy, and pour into desired container of choice. Keep refrigerated. Put the pot in the fridge overnight, and in the morning you should be able to separate the tallow from whatever water is left. Lastly, cut the tallow up and smoosh it into a wide-mouthed jar or container for easy access.
I used a cookie cutter and then shoved the leftovers down the sides of the jar. Keep it in the fridge, and it should keep for several months. Pretty tasty!
In summary, I would say that if you have any access to fat, it is definitely worth your while to render your own. To make this task easier, move the pot to one side of the burner so that only half has contact with the heat.
The simmering bubbles on the heated side push the fat to the cooler side, making it easier to skim. Do you have to skim stock? If you leave the lid off of the stock pot and keep from boiling the water, your proteins will be encouraged to coagulate link rise to the surface, where they'll dry out and become scum. From there you can skim them off. This is not disastrous, because the stock is still a perfectly lovely and usable foodstuff.