This is how I did it on my Nissan Bluebird so it should be similar
Jack the cab up on a level ramp or over a pit, remove fluid dipstick, get some rubber gloves on, get under cab and undo the drain plug then remove the base of the gearbox housing, in the base you will find a very thin flat filter, possibly under a plate in the base, remove this and wipe and residue off with a good cleaner discard the seal.
replace with a new filter and seal and re attach the base to the box.
go under the bonnet and using the correct fluid refill the box do not over fill once you reach the correct mark on the dipstick the first time start the engine and select 1st and reverse to get the new fluid around the box then keep topping up and keep moving the selector, until the fluid stops at the full mark
I don't know if there is a chassis cross member in the way of the gearbox base perhaps MR T could enlighten me??

There is no cross number.... and the rest of your advice is pretty good......... this time ha ha

This was the stuff I got out of my fairways box when I drained it.I've seen cleaner oil come out of a diesels sump.The gearbox seemed a bit smoother afterwards.