Usually when you buy a seal kit it will give you all the seals for the unit so you don't have to try to figure out which seal is the problem. Anyone with some mechanical abilities can usually handle rebuilding them. The most important thing to remember is keep everything clean as you do it. As a rule a little bit of dirt will take a seal out quick.
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When I first saw a drip under mine, I ordered a "Seal Kit" off Ebay, it was cheap and came with a printed diagram. However, while waiting on it to arrive, I changed the fluid with the Valvoline Hi Mile Trans Fluid with stop leak and seal conditioners, and now no more leak. If it ever leaks again, and probably will some day, I will replace the seals with the seal kit now hanging in the garage.
As typical with hyd. cylinder leaks, you will just have to remove, clean, and examine seals - the bad one will usually be obvious while looking at it. And, as mentioned, clean is key!