We do not need a book called 'The key to the Scriptures.' The key to the scriptures will be found in the Scriptures.
Three great questions to ask as you prepare:
1. How can I take the audience to the place this was written?
There is no direct line from God to us.
Hearers and writers. . . . . . . . . . . . . . . Readers (Us)
We have to go back to understand what it meant to them then before we will know what it means to us now. If the letter is from Paul to the Corinthians we have to go to Corinth in the middle of the first century CE.
2. What seems odd to me?
If questions appear to us they will probably be unusual. Try and imagine what would seem odd to the person reading this out of context for the first time.
3. What are the contexts?
Relationship between author and reader
Emotional Language and style
Within the book
Within the Bible
There may be more.
That done we need to ask one final question; what is this passage saying uniquely? What would the Bible lose if this passage was not in it?