Start with 5 RB's and/or WR's and place them sparsely on the field to the left and right of center. The coach will assign each of these players a number. The coach will stand behind the QB and raise his fingers to tell the players who is the open receiver. When the QB is ready he begins to drop back (or sprint out) at the same time all player in the field raise both arms high in the air except for the open man he only raises one arm. The quarterback has 3 seconds to find the open player (player with only one arm raised) and release the ball. If the QB can not find the open man within 3 seconds he must run the ball. At the snap of the ball the coach will yell out "One thousand one, One thousand two, One thousand three, GO". Later after QB's are getting the hang of this drill, the coach should add players in the field. Also the coach may want to discontinue counting aloud and just blow the whistle at the 4 second mark--indicating to quarterback you are sacked for a loss.
To improve QB's ability to view the entire field and locate the open receiver.
Make sure the QB is dropping back properly, ball high and near the throwing ear, foot work is proper, and weight is on his toes. Make sure QB is gripping the football properly. Check throwing technique, make sure QB is not over striding, ball is thrown from the ear, elbow of the throwing arm comes thru before the ball, points left toe and arm at the target so he does not throw across his body, on the follow through the thumb of the throwing hand should pointed down, the palm outwards and the pinkie should be pointed upward.