Combine cornflour and water to a smooth, thin paste in a pan. Slowly stir in the salted water from the vegetables that have been cooked - I find greens water the best. Pour this over the meat juices with a little of the meat fat in the roasting pan and stir well, gathering up all the juices and bits of meat, etc. Let this thicken, then simmer for a minute or two. Sometimes other additions are just not necessary, but for extra flavour crumble in an oxo or similar and I add a shake of Worcester Sauce.
Sometimes we need gravy when we haven't been cooking meat. I find it useful to chop an onion or two finely and brown in a little oil, then add the vegetable water and flour mix and crumble in an oxo, and add worcester sauce.
PS Adding wine or ale to gravy makes it really good!
These aren't traditional recipes, just what I have grown up with and find successful. Once you start making your own gravies and sauces, then you become quite expert at adapting them and making them better.