Consider two sources broadcasting identical waves in phase with one another. The sources create an interference pattern around themselves with bands of constructive and destructive interference.
The key to unlocking the secret of the pattern is this - what happens at any point depends on the path length difference, the distance from one source to the point minus the distance from the other source to the point. Call this path length difference DL.
Condition for constructive interference: DL = nl, where n is any integer.
Condition for destructive interference: DL = (n + ½)l, where n is any integer.