You negotiate a deal with the seller and then it goes to the lender to see if they will take less than what is owed on the loan. This could take 3 to 6 months to get an answer back and close. You have to be patient it will depend on how backed up the lender is.
Also, depending on the lender, there may be back taxes owed and the buyer may have to pay those, there may be a second loan and the buyer would have to pay something on that to satisfy that lender, you will probably have to pay for any required inspections, may even have to pay for closing costs, surveys and title charges. All short sales are different, this is just something to be aware of.
The other thing is that just because it's a short sale doesn't mean the bank will approve it. It could get denied by the bank and then you will have to start all over.