I used tea staining in my butterflies which have been mentioned before. It's great for giving an aged look to fabrics and fibres.
First, it works best on natural fibres - wool, silk, cotton, etc. It works because the tannins naturally present in tea act to hold colour onto fibres. Pre-washing your fabric is a good idea to get rid of chemicals from the manufacturing process.
Beside that, it's pretty simple - boil water, make strong tea. What I did was to make the tea a bit stronger than I would if I were drinking it. I used black tea (ie. ceylon, chai, earl grey, orange pekoe). Each tea is a little different and so your shade may be different also. ** You should test small swatches before staining a larger piece. ** The strength of the tea and how long you soak your fabric will affect how dark it stains as will the weight of the fabric. A lighter/thinner fabric will take less time.
You will get a more even colour if you make your tea in a container that will have enough room to move the fabric around, and stir it every few minutes or so. If the fabric is more tightly packed into the container you will get uneven colour, more like a tie-dye effect where the colour bleeds and fades. This can look really neat, but may not be what you want.
So, if you're staining a yard or so of fabric, fill a large pot or sink with freshly boiled water. A hot soak will work better than cold.
NOTE 1: It's best if you use an old pot that is not used for food.
NOTE 2: Be more careful about temperature with wool. Heat and friction can both cause wool to felt, which is not always what you want.
Drop in your tea bags, and let sit a few minutes. Remove the tea bags, and drop in fabric. Stir it around to make sure it's really wet. Soak for 10 minutes to an hour. Stir often. I find that after an hour or so it doesn't get alot darker. Remove from the tea bath, empty that out, rinse the fabric with clean water, and then dry. I wring it out and then hang it outside.