Using your owner's manual or your eyeballs, locate your radiator's drain plug. It could be anywhere along the bottom of the radiator, and will be either a screw plug, bolt plug or a petcock (simple drain valve). Be sure you have your used coolant receptacle in place under the drain before you open it up.
With your coolant catcher underneath the drain, unscrew it and let the coolant empty completely. If you have a screw or bolt type radiator drain plug, remove it completely. If your radiator has a petcock, open it all the way.
*IMPORTANT: Coolant can be very dangerous to pets. It tastes sweet to them but ingesting it can be fatal. Be sure not to leave any -- even a small puddle -- where an animal could drink it.