Remember making roux in class? You heat the oil or fat or butter in a skillet. Once hot, add the flour and whisk while it cooks. This will thicken up and is your roux. It will brown in about 1-2 minutes.Then add your liquids. In this case, broth or a combination of broth and milk. Stir as it thickens on a medium low heat.
If you need more, just remember to keep the fat to flour ratio even. Using drippings from a roast will make a fabulous gravy! Just add the flour to the drippings to make the roux, (whisking and browning) then add beef or chicken broth, depending on what meat you roasted, heating until thickened. Have fun!
You can saute sliced onions or mushrooms for the gravy. Or perhaps you have a special recipe that just needs thickening. Just remember your roux basics and you can thicken any sauce you want!