Go to a butcher or a grocery store that has a real meat counter and ask for several pounds of beef fat scraps. The butcher may look at you funny. Say what you want the fat for, and the funny look usually goes away.
Make sure it is understood that you only want BEEF fat. Pork fat makes LARD, which has a different consistency than tallow. Phone the butcher first, since meat cutters usually throw out all the scaps after the morning cuttings. Some places do not charge for fat scraps. If you're charged more than a nickle a pound you're being ripped off.
5 to 10 pounds of fat is a good amount to start with. Rinse it off with cool water, trim all the meat scraps off (use the meat to make broth or feed it to your dog - it will be fresh and will have been refrigerated). Chop the fat up into small pieces. The smaller the pieces, the better it will render, but it is tiring after a while, so I usually cut the pieces about the size of my thumb.
Fill a large pot - I use a stock pot or a canning pot - 1/3 to 1/2 full of fat and up to about an inch from the top with water. Put it on the stove over medium heat. Rendering tallow can be a rather smelly business, so turn on the fan in your stove hood, open a window, put a fan in the kitchen, or something.
Bring the fat and water up to a low boil, and keep it there for a couple of hours, stirring every 15 to 20 minutes. Skim off any foam or blood that may rise up. Be sure to add more water as it cooks down. Be patient. As the tallow and water cooks out of them, what's left of the pieces of fat will shrink up into ugly little greyish things called "cracklings."
Take the pot off the heat and remove the cracklings with a slotted spoon or a seive. If you really want to, you can render them again to get the last bit of tallow out of them. I usually just throw them out. Strain the liquid - carefully! - through a few layers of cheescloth into a large mixing bowl and let stand to cool. After a couple of hours put it in the refrigerator to chill.
Once it's chilled take it out and remove the white stuff on top: this is tallow.
The water underneath will be grayish and nasty, and a layer of gelatin may cling to the bottom of the tallow. Discard the water and the gelatin, and scrape the bottom of the tallow cake clean.
If the tallow is fully rendered, it will be firm, uniform in color, and smooth in texture. If, at room temperature, it is yellowish, semi-liquid, grainy, or oily looking, put it in a pot with an equal amount of water, bring to a boil, strain into a bowl, and cool again, and discard the water and impurities that settle to the bottom. You may need to do this two or three times to get all of the impurities out.
Wrap the finished cake of tallow in plastic wrap and store in the refrigerator.
It will keep fresh for a couple of months.
I do not know if there are any special tricks for making molded candles from tallow. I can only suggest the old "dip" method, wherein you dip a length of wicking into melted tallow (the tallow will turn yellow when it melts, by the way), pull it out and let the tallow harden, dip it again to add another coat, pull it out, etc., until you reach the desired thickness of candle.
Good luck with the candles, and let me know how they turn out!