Scouring cotton before dyeing with botanicals is an essential step. Before dyeing and mordanting you must scour away any build up that's within the cotton fibers. Whether it's a new cotton shirt that most likely has an artificial/chemical layer for softness or an old t shirt that picks up your own body oils, these particles need to be scrubbed away so that plant pigments can get into the cotton fibers. This process is just for cotton items, it can be used on hemp as well, but not for silk, wool, or any protein based fiber. Because you are boiling your fabric, it's important to know that your cotton item could shrink a little bit in this process.
Small batch scouring :
This process below is just for one to two items. I will make another post about large batch scouring.
You will need a large soup pot for your stove top that will fit your items in it. Next fill with water. Make sure there is enough room to stir the fabric. Add 1 tsp. of liquid dish soap like dawn. It can be whatever soap you hand wash your dishes with. Add soap to water, stir, then add your fiber. Heat up to a boil, then turn down so that it simmers for 1 hour. Pour out your liquid, it will most likely be a yellowy color, and now your fabric is scoured!
You can go right into mordanting your fabric if you are going to be dyeing with dye pigments that need a mordant, or you can dry your items and save them for another day.