You will have to follow googles tutorial. Here is a blog that explains it in detail.
depending on your integration you need to add accordingly. Like for instance you would add to your showphoto template the code to display the button where ever you want it to display.
For instance in showphoto adding this line in bold may work.
Content visible to verified customers only.
You would add the script call to your header. For instance if your using vbulletin you would add the script call to the header include template I suppose.
I have no idea what google plus one is or what it is suppose to do but that is what that tutorial says to do to add it.