There are multiple challenges when it comes to repairing leaky hydraulic cylinders. The first challenge is figuring out what size, model and design of cylinder you're looking at. Without that info, you can't order the correct seal kit or repair parts. In many cases part numbers or some sort of identification code is stamped on the side of the barrel or end cap. Whenever possible, write down every number and code you can find on a cylinder--or drag along the cylinder--when you head to town to get repair parts.
The second challenge is figuring out how to get the *$^#%! cylinder apart. If it's a simple cylinder where four long "tie" bolt's hold the end caps on either end of the barrel, it's easy enough to remove the nuts or bolts and use a hammer to tap one end cap loose. But if some engineer got cute and it's a tie-wire or internal snap ring-type or some other exotic cylinder that has no obvious way to get it apart--about all you can do is consult an expert. And if I'm the expert you consult, I'll probably have to look in a tech book, because I have trouble remembering all the tricks necessary to disassemble a lot of those funky cylinders.
Once the cylinder is apart, it's a matter of patience and time to remove all the seals and o-rings. The key is to move slow, pay detailed attention to EXACTLY how EVERY seal and 0-ring is oriented before removal. Sometimes it's really hard to tell the difference between the front and the back of some o-rings and seals, but trust me--high pressure oil will find the flaw if a single component is installed incorrectly.
Another challenge is deciding if the cylinder is worth rebuilding. There is usually a reason a seal or o-ring started to leak---maybe the piston or the inside of the barrel has a scratch or has worn thin. Maybe the rod is pitted, which eats away at seals every time the rod moves in and out. My attitude is that there's no gain in putting new seals in a cylinder that will just eat the new seals, but...the decision is not always left up to me.
Reassembly is sometimes a challenge, trying to get stiff new seals and o-rings squeezed into tight fits. Patience and lots of hydraulic oil or grease help. The hard thing to accept is that if you see even a sliver of rubber peel off as you assemble a cylinder, it's time to start over with a new seal or o-ring. As noted earlier, high-pressure hydraulic oil is really good at finding EVERY little flaw in a hydraulic cylinder.