Hello Mark,
I have a GTX 560 ti myself only the 2GB edition in SLI.
Its very simple you just buy a second card and it has to be exactly the same so you have to buy the GTX 560 ti 1 GB not the 2GB or something else or it won't work in SLI.
Just put the second GTX 560 ti on your motherboard in the second GPU slot if you have one. If you don't have 2 GPU slots then you need a new motherboard.
When you placed the second card on your motherboard you only need to put the SLI bridge on the cards. connect the power cabels and start you PC and activate nvidia surround in the nvidia control panel.
here is a youtube vid that explains it.