There should be a readme indicating what file is the seed file (what the file is encoded to).  There are a few decoders out there, here's a clip from Russel Cook's program readme. 

3.2 Decoding a file

• Make sure the "Decode" radio button is selected at the top of the dialog.
• Press the "File to Decode" button to select the RTE source file to decode.
• Press the "Key File" button to select the file to be used as a key in the
  decoding process.  This must be exactly the same file used to encode the
  source file.  If the file is different, you will be warned.  You will be
  prompted with the expected file name.
• Press the "Save As" button to select the output file name.  Once the file
  name is enetered, the conversion process happens immediately.  You will be
  notified of the success or failure of the conversion process.  You will be
  prompted with the original source file name that was encoded.

It's been a while since I've used anything else, each one should have a set of instructions.  It's relatively easy, but can be intimidating when you do it the first time.  

What we do in life, echoes in eternity.

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