In order to commit files one, they should be staged. SourceTree shows them in the "Files in the working tree" area.
Files must be drag&drop into the "Files staged in the index" area.
With a drag&drop files can be unstaged.

In order to speedup operations, 2 shortcuts could be introduced: one to stage one of more files, one to unstage them.

Using the mouse is really annoying. A developer could have an alternative.