For hard translations I like to work on paper because I can write individual words or letters on the original as I decipher them next to the word or sentence. I'll describe the process that way, but most of it could be done on a computer.
I like to lay the original text out along with two blank sheets of paper. If there is room, I will write a transcription of the German text in Latin letters on a copy of the original. The first blank sheet of paper is to re-create the scribe's alphabet, and the second blank sheet is to write the English translation.
Re-creating the scribe's alphabet is a crucial step, something that professional paleographers do. Write the Latin alphabet down the side of the page - as you work through the text, write the examples of how the scribe makes the letters, both capital and small, on the page. If he makes them a few different ways, write all the variants that you find. This will help you recognize letters in tough words.
|Scribal alphabet that I recreated for a translation.|
Now work through the text from the beginning, writing German words in Latin letters above the original script. Each time you decipher a new letter, write it on your alphabet page. As you start to get words, look them up and write the English on your translation page and start to massage them into English sentences. If you just can't get a word, write down the letters that you do know and leave the rest blank for now. Professional paleographers work letter-by-letter on tough translations, and you should too. I usually have to go through a text several times before I get most or all of the words. Sometimes I never get all the words and have to guess at the meaning of a sentence.
Finally, quality check your work. Read it to make sure the English is smooth and sounds like native English and not a mechanical translation. Think about whether each sentence makes sense in the context - if you have made a translation mistake, then a word or the whole sentence often won't fit the context. If you know a German speaker, you could ask him to review your transcribed text to see if you got the translation right.
Here's a sample transcription that I did of a West Prussian land document where my 5-greats-grandfather Gerhard Fast (1739-1828) was a mayor of a village in 1782 and affirmed the veracity of a real estate contract.
And here is my final translation. Notice that I have several words that I couldn't read for sure (e.g. insonderheit) and some that are not standard 21st-century German words (e.g. Leud instead of Leute for people). This is simply a reality when you are translating old texts. But I hope that this translation demonstrates that someone who starts with no knowledge of German can, with persistence and practice, translate a moderately difficult text.
Good luck and let us know in a comment how you are progressing.