It is implied that Dooku joined Sidious while he was still a Jedi Master. But before Dooku told the council he was leaving, Sidious had him (1) Assume the identity of a dead Jedi (Dyas) and go to Kamino and order a clone army, then (2) erase the record of Kamino from the Jedi files/archives.
That wraps everything up.
No...*sigh* wrong....Dooku as he was leaving the Order was recruited by Sidious..before Dooku left the Order Sifo-Dyas, a friend of Dooku's, had a bad feeling that the Republic would be in trouble in the future particularly war, hence he went to Kamino and ordered the Clone Army for the Republic....Sidious and Dooku found out about this and for a final initiation into the Sith, Sidious commanded Dooku to kill Sifo-Dyas since he was beginning to sense that the Sith were returning. Dooku murdered Sifo-Dyas after he had already ordered the Clone Army, making Dooku a fully flegdged Sith, and new apprentice to Darth Sidious. Sidious told Dooku to not worry about the Army, that it would "come to use in the future" but not now.
You're not seeing the full force of Palpatine's plan. He NEEDED a clone army and knew it ahead of time. Dyas had no motivation to order an army. He stayed true to the order and the council until he died.
Sidious on the other hand, knew that in order to take control of the Senate more fully, he would have to have an army to fight his war. But it could not be traced back to him. Dooku presented himself as Sifo Dyas (he was a Jedi anyway) to the Kaminoans and ordered the army.
Sidious knew that it would be discovered and be traced back...but you can't ask a dead Jedi questions, so his Sith identity remained safe.