When chicken is cooked, take it out of the roasting tin, cover it with foil to keep it hot and let it rest.
Tip out most of the fat from the roasting tin (with a chicken there may not be a lot - and you do need some to let the flour thicken the gravy). Put the tin on to the hob, and on a low heat, add a heaped tbsp to the roasting tin and stir this into the juices. It will go very thick and lumpy quite quickly.
Using the water that you have boiled your veg in (assuming that you're having boiled veg of some sort!), gradually add this to the tin, stirring all the time so that there are no lumps, and make sure you scrape up all the bits from the bottom of the tin.
Keep going until you have the texture you need, then season.
I sometimes add a bit of wine, or soy sauce (in which case don't use any salt), even Worcestershire sauce and sometimes a bit of cranberry sauce just to give a little bit (only a tiny bit) of sweetness. Just do what you think it needs to give it the right flavour.
HTH