An ice bath can be a relatively simple and easy way to chill your wort after you have finishing brewing. The primary issues with using an ice bath are needing large quantities of ice and a large sink or basin to place the ice bath and brew pot into.
What to Do
The best method is to place the pot into the sink or basin first, then fill with ice around the pot, and finally add water until it is almost level with the ice. This maximizes the surface area of the brew pot that is coming in contact with ice cold water.
What not to Do
Filling with ice and/or water first is not recommended, as trying to put a hot brew pot into the bath can be difficult and dangerous, with spills and burns quite likely as the pot slips and twists into the bath.
After the ice bath is prepared and the wort is cooling, occasional stirring with a sanitized spoon can speed the process greatly. When stirring, use great care not to be too vigorous and create bubbles, as this can oxygenate the beer, which will cause off-flavors farther down the line.
Gentle swirling will draw the cooler wort away from the sides and bring the hotter wort in contact with the cold metal of the brew pot. It is only necessary to stir every five minutes or so to keep the wort moving.
The entire process will take approximately thirty minutes to reach pitching temperature of ~75ºF/24ºC.