well you cant merge more then 2 at a time, it opens the documents side by side and highlights the differences so you can see what you are changing and can compare. The arrows at the top allow you to change from right-left or left-right, there really isn't anything to it. If the whole document happens to be highlighted its because the lay out is different, you need to use a program like "Notepad ++" to make the layouts on the documents the same so winmerge can show you the exact differences in the lines. Everyone seems to use the pretty print or pretty print with line breaks under the plugins/xml tools are of notepad ++