Do NOT add flour!!!!
no no no!!!
the only time to use flour as a thickening agent is to use it BEFORE the soup is made. it needs to be mixed with butter or oil in the beginning and cooked for a while before adding the liquid. If the soup is already made... us equal part COLD water and corn starch.
And... at a push, if you have a box of fake mashed potatoes laying around, add it a tablespoon at a time until it is thickened.
but when thickening anything... be careful. go in small batches, a little at a time and let it simmer/bubble for a minute or two before adding more.
if you add raw flour to a cooked soup, you will definately taste it.
edit: you can add flour/butter in equal parts to liquid to thicken, but it has to be a slow cook to give the flour time to cook. it is not a good idea to add it at the end of the cooking time.