Many cylinders can be repacked by yourself with a little knowledge, some basic tools, and the proper seal kit.
Depending on your cylinder, you can take a pin spanner (if there is a small dowel hole), a hook spanner (if their are notches), or even a drift punch, or pipe wrench and unscrew the cylinder gland to pull the rod out. At this point you'll have access to the piston, which may need to be unscrewed from the rod. There will be a number of seals, backing rings and wipers that will all need to be replaced on the cylinder and gland, all of which should come in the correct seal kit.
take a look and the exploded view
Technology of Hydraulic Cylinder - Magister Hydraulics
Removing some of the old seals can be tricky and having some good O-ring picks will be a must. When installing some of the new seals, especially the solid piston seal, allow it to soak in hot water. This will make it more pliable.
The guide rings in the gland can also be tricky to get out, and install. Again, a good set of picks to help get them out. Patients and good technique to get the new ones in.
This is all assuming there is no damage to the cylinder or rod. If the inside of the cylinder, or the rod is scored to a certain degree, a new seal kit will not last long without repairing the damage first.