place some of the cooking liquid into a pan, with a stock cube (or gravy granules if you prefer) if you haven't used one earlier, bring to the boil, then reduce the heat and simmer, stirring until the stock cube is dissolved.
If you are using gravy browning add that now
Put one tablespoon of cornflour into a cup and add a little cold water, stirring to a loose paste, if the cornflour feels resistant to stirring add a little more water until it loosens up.
Remove the pan from the heat and allow any bubbling to subside, then add about half of the cornflour mixture stirring all the time, return to the heat and bring back to the boil. If the gravy isn't thick enough repeat the process with the rest of the cornflour mixture always remembering to remove the gravy from the heat before adding the cornflour otherwise it will go lumpy.
Remember that gravy is not scary - if it goes lumpy you can either whisk the lumps out or you can put it through a sieve. If it is too thick, add more water, if it is too thin, make more cornflour paste and add that.
If you don't have cornflour then you need to do things slightly differently. Place a tablespoon of cooking liquid into a saucepan, add a tablespoon of flour and stir them into a paste. Slowly add more stock until the gravy is the required consistency and don't forget to add your stock cube.
If you prefer onion gravy, slice an onion and fry it in a little butter and oil until it is translucent. Sprinkle the flour/cornflour over the onions, stir it in and then add boiling water stirring all the time. Add stock cube and browning or gravy granules keep stirring until it is boiling and then reduce heat and simmer.
if not have a look at the following link