If you want to sell your home in a short sale, then you will need to prove to the mortgage company that you are no longer able to make payments because of hardship. The hardship you face should be documented; if that means you lost your job, have proof of your termination, and you should also show financial proof of being unable to continue paying. With your attorney, this information is gathered and submitted to the lender.
Once you've been allowed to start a short sale process, you can list your home and wait for bids. A short sale will be allowed only if your lender approves it. That means that if the lender doesn't think you're getting a good offer, you probably won't get an approval to sell the home and will have to look for a sale at a higher amount. Most of the time, you and your attorney can speak to the lender to determine what amount the lender will accept, which helps avoid a lengthy wait for an approval.