Take drippings from whatever meat you cooked. My pictures here are from a beef pot roat, which is why it turns dark, but you can make this with any meat. Just cook in a pan or pot that will catch the drippings (which is really a fancy word for melted fat that cooks off). You can also use the water from a roast you cook in a slow cooker.
Put drippings into a metal skillet (do not use non stick, because you really want to use a wire whip, which will scratch your pan - opt for stainless or cast iron). Taste it right now to make sure it has enough salt, pepper or other flavor. What it tastes like right now will be what your finished gravy will taste like, so be sure it's good or add any seasonings at this moment before you go any further. I like my gravy a little salty.
Put it on to heat on medium/high.
While drippings are heating, fill a mug or 8 oz glass 3/4 full with hot water. Measure a fork full (yes, fork.) with corn starch. This is all you need, don't add more or your gravy will be gummy.
Whisk with the same fork until it's absolutely smooth and watery.
With the whisk in one hand and the corn starch in the other (I know, it doesn't show in the picture, but trust me, be ready to stir!) slowly drizzle the corn starch liquid into the boiling drippings. Adding it slowly while stirring will prevent any lumps.
Whisk like crazy and do not stop whisking until the gravy thickens. Make sure you are scraping the whisk on the bottom of the pan, and hitting all areas of the pan.
When your gravy thickens enough so that the bubbles seem like they have to struggle to pop, your gravy is done.
Take it off the heat IMMEDIATELY.
Pour into a gravy boat and enjoy.
Note: Gravy does not keep. It will turn solid, so unless you're making Bubble&Squeak the next morning, there is no good reason to keep it with leftovers.
(If you're curious as to what Bubble&Squeak is, tune in the day after Thanksgiving! I'll post a tutorial.
You might be surprised, like I was, to find you've been making it for years without knowing the British had a name for it!)