Well there are basically two types of leasing. The are wet leases and there are dry leases. Dry leasing, is when a company leases only the aircraft. A wet lease is when the company leases the aircraft and the crew. Crew leasing is when a company borrows crew from another airline to assist in their own airline. This is done under a contract. The crew is given uniform, etc by the company that has leased them. Flight crew lesing happens quite seldom i would say. The exact same procedure is done for the lease of a flight crew.