Your lever neutral position needs to be centered. The steps listed below assume that all of your linkages, pumps, centering mechanism, etc. are in good shape with no excessive wear. All wear issues should be dealt with prior to adjusting, or else hitting the neutral position may not be repeatable.
Start by raising the machine so that all wheels are off the ground.
There should be a pair of rods that run from a bell crank levers on the shaft that the steering levers are mounted to, and run back to levers that are mounted on the pumps. These levers control the swash plates in the pumps.
The rods have rod ends on both ends, secured by jam nuts. Loosen the jam nuts so that the rods my be rotated inside of the rod ends. Hopefully, they are not seized.
Start the engine, and note wheel movement, indicating a need for adjustment. Rotate the rods until wheel movement ceases, and tighten the jam nuts. Move the steering levers forward and rearward, allowing them to return to neutral and see that the wheels do stop. Tweek the adjustment if necessary.
HTH - SR