I am no expert at it but the way I understand it is you need a acid flux that eat the oxides on the stainless so you get the proper bond to the stainless.
A good article on it. Scroll down to Soldering.
BT - Brazing and Welding 304L Stainless Steel
You need to avoid any solders with cadmium in it.
Be careful when soldering stainless as the fumes are very nasty for your lungs due to the acids in them.