OK, it's really easy. It all depends on what you intend on what you're using the gravy for, but let's say that we're going to make sausage gravy for biscuits.
Fry up sausage. Get a sausage that has some fat to it because that's what we're looking for. (Jimmy Dean doesn't work very well for this) Crumble it as it cooks.
Remove sausage from skillet and set aside. Put heat about med-med high. Put in 2-3 TBsp of flour. (This all depends on how much fat one has left over) Stir the flour into the fat and let it cook down for a bit, 3-5 mins. Make sure that all lumps are gone. It should have the consistency of glue. Stir quite a bit, but not necessarily constantly. (This is called a "roux". It can also be made with butter or any other fat) Add in salt and pepper at this time, about a teaspoon of each, unless you're using bacon fat.
Add in milk, slowly, typically about a cup. Let it cook for a while, again, stirring and watching for thickness. (5-10 mins, maybe more ) Add sausage back in once it gets to desired consistency. Pour or ladle over biscuits.
I also do similar with chicken gravy. However, I separate the chicken juice from the fat ( a gravy separator is essential for this unless you want to wait overnight. If you want to wait, put all the juices in a container and refrigerate. Separate fat from juice when congealed. ) I make the roux with the chicken fat & flour. Once the roux is ready, I pour in the reserved chicken stock and maybe add in some milk if the gravy is still too thick at this point. Depending on how you season your chicken, you may want to taste the gravy before you add any salt or pepper. The key is that you want the gravy to be a little on the "thin" side, initially, until it cooks down and obtains the right consistency.
The cheating way is to make a roux, add in milk or water and chicken or beef base. Add in pepper. Chicken and beef base already have a lot of salt, so be sure to taste before adding in any additional salt.
A note: I cook by taste, texture, look and feel, so I don't really measure anything.
You'll have a few mistakes until you get it right. You may or may not get it right the first time, but that all depends on your cooking skills.