Slide the STX38 mower deck under the mower. You will see three brackets protruding upward from the mower deck. To ensure you position the deck correctly, place the deck so you have a bracket on the right side, left side and front.
Place the mower deck onto blocks to lift it up. This makes it easier to reconnect the mounting brackets.
Slide the deck forward until you can place the drive belt on the deck around the clutch pulley extending from the bottom of the engine. With the pulley mounted, slid the deck backwards until the bolt holes in the brackets line up with the bolt holes under the tractor.
Secure the mower deck to the underside of the tractor with the original bolts and a wrench. Each of the three brackets has two bolts.