Soak them separately in room temp water with a few drops of dishwashing soap overnight to hydrate them - that makes the fibers stronger.
Next day wash then with quite warm water and a plain detergent without additives, you decide by hand or in the washing machine. Dry in on a line or in the dryer on cool or room temperature. DON'T USE HOT! It sets the stains.
Inspect the stains. If they are lighter you are doing the right thing...try again. If they are the same, ask for advice here!
Because, if they are organic, like dried cat pee, the treatment is different from that for oil or grease .