Brew some tea. You brew the strength of the tea, depending on how dark you want to dye your fabric. Also it depends on how long you leave it in the tea as to how dark it can get.
Dampen your fabric and do a test with a small swatch. Always make sure your fabric is wet before you put it in the tea. When you have the fabric the colour you want, take it out and put it in cold water. Rinse until the water runs clear. Dry your fabric.
I have done this a few times and it works really well.