lol i had a friend who tried fixing his own forklift brakes (old hyster)
the cones stumped him for 5 hours before he called me

i got the wheels off in 10 minutes
he promptly cursed me out lol

i tap (not beat) my screwdriver into the seam of the cone after theyre a lil loose
that spreads em enough to get them off easily plus they usually clamp onto the screwdriver and you can pull them right off

i keep a threadchaser or die handy to fix any "misses" lol

this is a pretty "normal" type differential axle, I guess they don't teach this stuff for sit down Raymonds? I think if you don't know how to get into the brakes, maybe you should "apprentice" awhile with a tech that has a bit more experience, before you try and stretch your abilities until someone else gets hurt from a mistake in the brake job?, just my $.02 USD.
Jack the truck up, use something to hold the seat down (to take the parking brake off) take the axle out, (use a common screw driver and pliers to remove [unscrew or pull off] the cone shaped washer/spacer around the axle studs. being very carefull to not strike (and so to damage) the axle to hub studs, hit the center of the axle with a big (sledge is what I use) hammer to make a fraction of an inch looseness between the axle and the cones.
Take off the nuts that hold the bearings in, then if you still need to, you might have to let pressure off the bleeder, or un adjust the wheel brake adjusters.