You have to remove the fuel tank, seat assembly and all the supports under the seat frame assembly, brake linkages, and lift off steering clutch compartment cover. Disconnect steering clutch yoke linkage.
The clutches are held in by a row of bolts around each side of the clutch assembly.
You need to chain up the blade before you start and allow room to move the tractor backwards or forwards a foot or two, so you can access all the clutch retaining bolts.
You move the tractor a little at a time, using a bottle jack against a grouser, to turn the clutches so you can access the retaining bolts. You can usually lift steering clutches out utilising the brake band. Connect a lifting cable or chain to the band. Make sure the band is centred on the clutch, and the chain/cable is acting to tighten the brake band as you lift.
The D4 has the brake bands secured to the bottom of the steering clutch main housing, these bolts must be undone from the underside.
You may have to remove the canopy or part of it to allow the clutch to come out.
Do the bearings in both clutches while you're in there. If there are 2 similar components in a Cat (one each side), they will both generally be in the same condition, so it pays to do both at once.
Check the bevel gear/cross-shaft bearings as well, they need to be in good shape. If the bearing races show serious wear, it pays to replace them.
The powershift D4D is a nice little tractor, it was designed as a construction machine. Good to hear you got a good buy, now comes the exciting part - pouring a whole heap more money into it, so it provides reliable service!