With any leather saddle, the important thing is: don't ride it when it's soaking wet.

Important to understand that not every leather saddle is the same. That's especially true of an older saddle. It is entirely true that a Brooks saddle can tolerate a lot of exposure to the elements without visible damage. It's kinda like a cat with its proverbial nine lives; in OP's case we don't know what this saddle has already been through. It may be on its last life.

Applying Proofide to the top and bottom is the correct treatment for some, but not all saddles. If the saddle has dried out (as a result of having gotten soaked and dried repeatedly, which leeches a lot of soluble material out of the leather) then Proofide will do some good. If the leather is still supple, Proofide may soften it to the point that it will stretch permanently.

SO my advice for the OP: you're going to have to make up your own mind about what your saddle needs. None of us has seen it, and none of us can offer specific advice. What you decide will be either the right thing or the wrong thing, and you won't know right away. Sorry, it's a learning experience.