A lot of time is required for that sort of thing. I guess the first thing is to figure out context, or at least best you can. What they're talking about is important, it can give you clues about what kind of words to look for, maybe military things based on the 1864 date. And then just look closely at the letters themselves, try to pick out recognizable shapes like g or e or l or t. Try to piece together words. Until you get the hang of the handwriting, context and going slow are your best friends.

Get some decent lighting, so you can see. Look at a sentence, type the words and letters you can make out into any word processor ( I kinda like just using Notepad). Where there's something you can't make out, give it some brackets ( ) and type hypothetical text into it and a question mark, even if it doesn't make sense or even a word ( Galambassian?). Then go over the original again, and some more words will likely be clearer. As you add more to the transcription, and get used to a particular script, the gaps will tend to fill themselves in, like"Harry came knocking at the ( joor? ) last night". Capital letters are sometimes odd, but usually you can find a word you recognise, so if for example you don't recognise the capital letter in "( )ight", you may be able to see recognise it in " ( )ichard Green " as an R.

Don't be in a hurry: if you can come back to an illegible manuscript again and again, you can usually slowly fill in the pieces.

Post it if you can. We can take a crack at it.