If you are in your home directory and you want to watermark a file in the Documents directory, you could plan out the command like this:

watermark [because that is the executable to be used]

-v [so that you can get as much feedback as possible]

-I [because you don't want a new file you just want to watermark the existing one]

-f [because you only want to change the one file not the entire directory]

filename.pdf [the name of the file]

-i 123456 [to set the identifier to 123456 - this is a required setting

-l Private [to set the sensitivity level to "Private"]

Now at the command prompt enter all of these components in order: