It is actually rather easy to do this.  At the tip of your facet, you have a piece that screws onto or into the end of it, this is the aerator.  Unscrew it and it should have it’s flow rate imprinted on the side.  A flow rate of 2.2 gpm (gallons per minute) is standard.  For low flow, go lower, such as 1.0 (or even 1.5) gpm or lower.  The below link can help you determine proper fitting (male or female and proper size): http://www.conservationwarehouse.com/faucet-aerator-faqs.html

You can find eco-friendly aerators at Home Depot, Lowes, most hardware stores, and on-line at Vine.com (my new favorite website!).

Screw your new eco-friendly aerator in, and you’re done!  Happy saving water, AND saving money.