Two or more persons came to an "understanding" to do a criminal act... When evaluating the prospect of criminal allegations of conspiracy, consider whether some sort of agreement (formal or informal) was in place
The accused knew of the criminal purpose and willfully joined in...
Any of the conspirators knowingly made an overt act to advance the object of the conspiracy... This "agreement" needs to be followed by the commission of any overt act - in furtherance of the conspiracy.