Begin with a good broth. See tips on vegetable broth
or making a chicken broth
. And if you don't have a broth to hand, you can always use broth cubes. Just don't start with plain water. You can top the soup up with water as you go, but starting with a good broth is the secret to the taste of any soup.
[Option -- Rice] If you want to add a handful or two of rice, now is the time to do so. Always use way less rice than you think is needed, as it will expand a good deal, and this is a recipe for soup, not risotto. Cook this for about 15 minutes and check your broth -- if it has become too thick, add more broth or water.
Add the chicken -- it can be either leftover chicken chopped up, or chicken from making the chicken broth.
Add the vegetables. They can be spoonfuls of leftover vegetable in the fridge, handfuls of frozen veg from the freezer, or tins from the larder. Vegetables can include: corn or peas, or any of the following, chopped: turnip, parsnip, carrot, potato, asparagus, onion, garlic, cabbage, cauliflower, celery, etc.
[Option -- Pasta] If you don't have rice or potato to hand, but want a starch in the soup, you can use a few handfuls of pasta. "Shapes" are best, but if you only have spaghetti, by all means you can use that, broken up. If you use pasta, it may thicken your soup, so you may need to add more water or broth.
[Option -- Thickening the soup] Anything starchy will thicken a soup: rice, pasta, potatoes. Leftover mashed potato also make a wonderful thickener.
Let the soup with the vegetables in it simmer covered for half an hour or so (longer if you are starting with fresh root vegetables such as potatoes or carrots, as you'll want to cook them until they are tender.) Actually, there is almost no such thing as simmering a covered soup too long. Check to see that there is enough liquid in the soup, if not, add either more broth or water.
When all the vegetables are soft, you can add the herbs. Ideally, you should add herbs a bit before serving something -- long enough to let the flavour get into everything else, but not so long as to destroy the flavour.
You can use herbs (dried, fresh or frozen) such as thyme, sage, rosemary, parsley, etc. Some herbs such as thyme and rosemary, if dried "intact", have tough stems and leaves that will never soften even if you simmer the soup for two days -- very unpleasant getting those caught in your teeth. Either avoid these and use the dried ground kind, or put them into a tea ball that can easily be fished out later.
Now check the taste, and adjust with salt (or salt substitute) and pepper. A soup will taste "flat" if there isn't enough of a salty taste to it. See "The Taste Areas of the Tongue." However, if you need to cook low salt for someone who will be eating it, and don't have a salt substitute to hand, then do so and put out a salt shaker at the table for the others.
Simmer for another 10 - 15 minutes before serving, or if you are adding dumplings, do so before this final simmer.
[Option -- Dumplings
] If you wish to add dumplings, do so just after you adjust the soup with salt and pepper. Then simmer covered for about 20 minutes.