To enter a new killfile: Click 'New', enter the text into the three boxes, and then hit 'Add/Replace'.
To edit an existing killfile: Select the killfile from the list, and it's details will appear in the text boxes. Edit them, and then select 'Add/Replace' to confirm the changes. If you wish to cancel the changes, then instead either select 'New' (ready to enter a new killfile), select another killfile entry (to edit it instead), or simply close the window.
To delete an existing killfile: Select the killfile from the list, and click 'Delete'.
To duplicate an existing killfile: Select the killfile from the list, and click 'Duplicate'.
Header: This is the header you wish to kill on, eg, 'From:'. It should be an exact match (ie, this is not a substring search), but it is not case-sensitive. Alternatively, leave blank to kill on any header. The colon will automatically be added if you forget to.
Text to Kill: If the header specified contains this text, the message will be killed. This is a substring search, and it is case-insensitive.
Newsgroups: The newsgroups you wish to set this killfile for. This is a substring search, so you can do 'comp.sys.amiga.' for all the newsgroups that start off with that name, and leave blank to kill on all newsgroups. This is case-insensitive (but all newsgroups have lower-case names, anyway).
Expiration: You can choose to either have the killfile never expire (it stays until you delete it), expire a certain number of days since it was created, or expire since a certain number of days since it was last used. If one of the latter two, enter the number of days into the adjacent text box.
This is a special type of killfile. Often, you will see people posting adverts or other off-topic spam to a newsgroup, but sometimes they will put a number as the last word in the subject, eg, 'Make lots of money 774'. If you enable this killfile, then Newscoaster will check the last word of the subject of each message; if it is a pure number, with three or more digits, then that message will be killed (the reasoning being that a one or two digit number is more likely to be an on topic message). Of course, if there is an on-topic message with the last word on the subject being such a number, then this message will still be killed, so it's up to you whether to use this feature or not!
When you are done, close the window and the changes will automatically be saved to disk.
Also, remember that there is a special additional killfile on the number of lines in a message - see the Edit Newsgroup window for details.