Jus is just beef stock poured into the degreased roasting pan, brought to a boil to deglaze the pan (scraping the stuck on bits as you go)--that's it. For best results use a good homemade beef stock. You can also deglaze with red wine and add the stock to that.
You can cheat this easily if you're cooking the roast outside and would rather not deal with a pan. If all you have is canned beef stock mix it 50-50 with canned low-sodium chicken stock, a sprig or two of fresh parsley, a smashed small garlic clove, a splash of red wine, a few black or white peppercorns, a tiny pinch each of marjoram and thyme, and a small piece of celery. Bring to a boil then simmer, covered, 20 min. Strain out the solids: reserve.
When your roast is done rest it. 5 min before you plan to carve return your stock to a simmer. Trim the end off the roast and include a bone if possible. Stick that in your pot of jus, bring it to a boil, cover it, the simmer strongly while you carve. Remove the hunk of meat and bone; serve.
A bit more involved: Get some good meaty beef bones (oxtails are good; if the bones are large chop them with a meat cleaver first), smear them with olive oil and a little tomato paste, put them in a pan with a 3-inch piece of carrot, a whole onion chopped large, a couple 3-inch pieces of celery, and a pinch of salt, and roast them in an oven inside (or on your kettle). Put a little water in the pan at the outset and add a little periodically so the the drippings don't burn. Roast at 350-375 for 90 min-2 hours, stirring quickly when you make a water addition, or every 15-20 min. Dump the contents of the pan into a pot, deglaze the pan with wine or water and add that to the pot along with a smashed garlic clove, a couple sprigs of fresh parsley, a few black peppercorns, a good pinch of thyme, a piece of bay leaf, and a little pinch of marjoram. Cover with water, bring to a boil then simmer 20 min. Strain out the solids, pressing a bit to extract juices. Degrease. [If you are using a roasting pan or drip pan for the roast pour off the grease in the pan, deglaze with wine or water, add this stock, bring to a simmer scraping up the stuck on bits.] Adjust salt; reheat; serve.
A different but common approach using a shorter roast time and longer pot time: Roast 3lbs of meaty bones alone in a pan in the upper third of a 450 oven, turning occasionally, till well browned, about 35-45 min.
Meanwhile, dice 1 onion, 1 celery rib and 1 carrot and saute them in a few T of oil in a large pot, stirring often, till the onion browns in spots, about 15 min. Stir in a T of tomato paste, move the pot off the burner; reserve.
When the bones are roasted put them into the pot with the vegs. Pour off the fat from the meat's pan and place over two burners over high heat. Deglaze the pan with water or wine, scaping the bottom well so that all the stuck on fond dissolves. Add this to the pot with the bones and vegs then pour in enough water to cover by 2 inches.
Bring to a boil over high heat, skimming off the foam that rises to the top.
Add a few cloves of smashed garlic, several parsley sprigs, a thyme sprig (or a few t of dried thyme), a couple t of dried marjoram, a bay leaf, and a t or 2 of peppercorns. Reduce the heat to low and allow to cook at a bare simmer for at least a couple hours--but you can go longer, all day if you wish.
Strain the stock (do not press on the solids) and allow to sit 5-10 min for the fat to rise. Skim off the fat, cool, reserve in the fridge or freeze. (Alternatively, cool the stock after straining then fridge it. The fat will solidify on top when the stock is cooled. Remove the fat then return the stock to the fridge, or divide into containers for the freezer.
To make jus if you are using a roasting pan or collecting drippings below: Heat several cups of your homemade stock in a pot on the stove to a simmer about 10 min before you expect your roast to be done. Cover, keep warm over very low heat.
While the roast rests, pour off the fat (only!) from the pan (you can also pour the entire collected drippings into a fat separator then pour off the fat from it) then deglaze the pan with a little water or wine, or simply use a little hot stock from the pot on the stove. Mix the reserved de-fatted juices plus the deglazed pan contents into the pot on the stove, stir, adjust seasoning if necessary; serve with the carved roast.