What you want to do is of course discontinue the use of your dryer. Shut off the 30 amp dryer breaker. Disconnect the dryer outlet. Now look at the wires. There should be a red wire, a black wire , a white wire, and a ground. To be able to operate a 1000 watt hps, do the following:
1.) Install a junction box (a metal box used to make wire splices) with the #10 dryer cable on one end and # 12-2 with ground coming into the other side of junction box.
2.) Take the black wire of the 12-2 and with a wire nut splice it with the black wire of the dryer cable # 10
3.) Take the white wire of the 12-2 and again splice it with the white wire of the dryer cable #10
4.) If there is a ground wire, Do the same. (make sure there is no bare wire coming out the bottom of your wire nuts as this will cause a short when pushed into the j box) better yet when finished splicing, tape the wire nuts with electrical tape.
5.) Take a wire nut and nut the remaining red wire. Thats the other hot wire. Unless you want 2 circuits for your grow, if you do, repeat steps 1 thru 4.
6. Now take and simply wire your outlet wherever you like. Black wire goes to gold screw. White wire goes to silver screw, ground to green screw.
Follow these instructions step by step and you will be fine.... I would reccomend only using this solely for the 1000 watt hps. Fans, heaters ect. should be used on another circuit as to prevent overloading. Use this circuit only for the 1000 watt hps...