To compare two files, select Compare from the Files menu (F9 by default).
Click on the first Browse button in order to select the first file. Click on the Read only check box if you want to protect the file from accidental changes. Do the same for the second file. Click on the White space must match check box if the two files are required to be exactly the same otherwise any white space (spaces and tabs) in one file will be allowed to match white space in the other file. Also blank lines are only significant when this option is selected. Finally click on the Compare button in order to open the two files and locate the first difference. The lines with the first difference are marked by red arrows in the margin and the caret is positioned at the appropriate point on each line.
In order to merge two files, it is a good idea to make one of them (the first, say) read only and to edit the second file accordingly.
To continue the search, place the text cursor on matching lines in the two files and then press Ctrl+F9 to find the next difference. Alternatively pressing F9 again restarts the search at the beginning of both files. This is recommended when merging two files.