Some of the first steps that you need to take is get an authorization to release information from the seller. This will allow you to find out from the lender the loan status specifics. The question hinging you need to ask is "will you entertain a short-sale?". From that point the ball is in their court on yes or no. If they will, then ask what type of information that will need and the major things will be:

1) Contract with seller
2) Seller's personal financial info---pay stubs, financial status, etc... Anything you can do to present the situation in a much less favorable light is to your advantage. Just nothing but the facts but also that reflects the true financial distress of the seller.
3) Net sheet of what lender would receive.
4) Detailed repair list of what it would take to get the property A-1 shape
5) Letter justifying your offer with repairs needed, current market conditions, solvency of seller, etc...

This is a start for you and the entire short-sale process can be comprehensive at times. However, this should start you in the right direction.

Thanks
Scott