If God ordains everything which comes to p, how is it that He is not the author of sin?

The only way to answer this the way the WCF did, would be to acknowledge that man stands condemned before the Lord. If we start out as guilty, then God can simply allow the guilt and the ociated behavior to continue, because man does not hold fellowship with God, and has no capability to do so, on his own. It is only with the regeneration received from the Holy Spirit and a continual working in man's heart, that he does hold that fellowship with God through our Lord Jesus Christ. Since He would be fully justified in leaving man in his sins, I believe the WCF provides sound doctrine. He guides the footsteps of the ones He calls unto Himself, and He also guides the footsteps of the reprobate, though they do not realize it or care.