Bacon jus is collected from the drippings of cooked bacon. At work after cooking the bacon in the oven I place the par-cooked bacon in a perforated insert on top of a deep (4-inch) hotel pan. I let it sit there for a while, at room temp, until the dripping is done. I then put it through a sieve into a bucket, letting the drippings sit at room temperature for a few hours until it has settled. I then place it in the fridge over-night. The next day I scoop up all the bacon fat from the top (about 90% of the drippings), and collect it for further use. At the bottom will be my gelatin like bacon jus. This jus will be very salty, so when you use it there is no need to add salt. It's good for mixing in soups or sauces, I will collect it over time until I have enough to do something with it. It can take a while, as with 10kg of bacon I will get maybe 2 cups of jus (if I'm lucky).
If you are doing this at home simply make sure to put the drippings into a small container, so that there can be a good sized layer of jus at the bottom to scoop up. If the layer is too thin (your collecting bucket too wide) then it will just stick to the fat and be hard to separate. If you don't cook the bacon (or par-cook it) in the oven, simply save the drippings from the pan.