You are referring to the hydraulic PTO valve mounted on a bracket just behind the Operator's seat.
If you look at that valve where the lines enter the sides, you should see the words IN and OUT. Begin by installing the bracket/valve unit to the tractor but do not tighten the bolts down. You want the valve to be flexible at this point. Remove the mower deck and jack the tractor in the air and use jack stands to support this 800 LB machine.
If you slide underneath with a trouble lamp, you will find the Travel/Lift valve under the checker plate floor. You will see a hose coming from the output of the pump to the IN side of that valve. After draining the hydraulic oil out of the reservoir, you need to remove that hose from the fitting on that valve.
The line coming from the IN port of the PTO valve gets connected to that hose. The line coming from the OUT port of the PTO valve gets connected to the fitting on the IN port of the travel valve. Once you have both ends of each line mostly screwed onto their respective fittings, you can now tighten the valve bracket solidly to the tractor. Then, tighten all lines to the fittings. Do not use any sort of thread sealer or teflon tape. It is not needed nor wanted.
On the bottom of the travel valve, there is a hex shaped pipe plug. Clean the hex hole out thoroughly and then use either a new or very good Allen wrench to crack that plug loose. You only get one shot at this because if your Allen key is junk or not inserted 100 percent, it may spin and ruin the pipe plug.
With a clean bucket under the valve, remove the pipe plug. Start the engine and the fluid will pump out into the bucket. When you see spurting, shut down the engine. If you have been careful to keep the oil clean, you can reuse it. If the oil is more than two years old, then replace it with fresh 15W40 motor oil or 20W50 motor oil. If you have a tractor with the tank under the battery, then fill that tank to six inches below the top of the filler neck.
If you have a tractor with the front mounted tank, then fill that one to an inch and a half below the actual top of the tank. This is to allow for oil expansion.