Patcher creates patches by diffing your local copy of the project against the repository. This is done automatically after preparing a message, so you shouldn't normally bother to do it manually. There are however situations in which you need to diff your project again, for instance if an error occurred during the initial diff (see More On Commands), or if you suddenly decided to further modify the source files.
The way to regenerate the patch manually is to call
patcher-mail-diff from the mail buffer. This function is bound to
C-c C-p d in this buffer.
When possible, Patcher also tries to check that your project is up-to-date with respect to the archive, and will inform you otherwise.
In the remainder of this section, we describe the different ways to customize the diff process and the appearance of its output.
By the way, (re)generating the patch does not necessarily mean that it is directly inserted into the mail buffer. This also depends on the ChangeLogs behavior (see ChangeLogs Handling).