Place the boom in a safe position, (most of the way down, but not all the way, and where it cannot possibly fall)
Bleed off the hydraulic pressure. (wiggle the controls with the engine off)
Remove hydraulic hoses from the cylinder.
Remove upper and lower pivot pins.
Remove gland from upper end of cylinder. (it unscrews, right-hand threads) You will need a spanner wrench, like this:
spanner wrench
Remove gland and piston rod assembly from cylinder housing.
Remove large nut from end of piston rod. (this may require a 1" impact air wrench)
Replace inner and outer seals and rings on piston, as well as the seal and rings in the gland.
Reassemble in reverse order.
Note: Lubricate all seal/ring surfaces before reassembly. Place tape over threads to keep from damaging new seals/rings. Remember to remove tape before screwing parts back together.