Well if you've got lots of juices from cooking a joint or poultry, you need to strain them off and scoop off the fat from the top. You can then put this in a pan, or use the dish that you were roasting in, to make the gravy. Depending on how thick you want your gravy you need to add flour - it will have to be trial and error I think until you know what you like. I tend to use a heaped dessert-spoon of plain flour. Blend it into the meat juices so that there are no lumps, and then add a very small amount of vegetable water (from cooking your vegetables) or plain water. Mix it in until there are no lumps, and then add a bit more water. Mix again and continue adding water to the juices and flour until you have a smooth liquid. You can then start to cook the gravy either in the pan or roasting dish, but start off slowly and keep stiring as if you are making a roux - you don't want the flour to lump. If you find that the gravy is too thin, mix some more flour and water together and then add it to your gravy. If it's very pale looking you can then add some port, brandy or sherry to help add some colour, or some marmite but be careful how much you put in as it's salty.
If you aren't cooking a joint of meat or a chicken I'd use gravy granules!