Sit down at your drum kit in the position you normally would play. Look at what position your feet are in when pressing on the pedals--this will help determine the angle at which you should place the double pedal.
Move the hi-hat pedal away from your kit, and take the single pedal off your kick drum. Attach the first pedal of your double kick pedal to the kick drum in the same position you had your single. Place the second pedal where the hi-hat pedal once rested.
Sit down in front of your kit and play each pedal to ensure you're comfortable with their positions. Adjust the angles of the pedals to suit your needs. Because the second pedal is attached to the first with a stiff rod, you might need to rotate the kick drum slightly to get the pedals into their correct places.
Place the hi-hat directly to the left or right of the left kick drum pedal. Which side the hi-hat sits on will be determined by the space permitted by your kit, and your comfort and style as a drummer. For example, most hardcore and metal drummers will keep the hi-hat to the left, where it is harder to reach because the double kick pedal takes precedence in that style of playing.