You could just load the negative in the holder using the glass plate as a backing. I do this with a 10x15 plate camera I use. I'd suggest trying it out with an old negative, but you wrote it's your first time trying LF, so if you don't have one available maybe cut a piece of paper or some plastic to size, then try loading that, and check that it doesn't fall out when you pull the darkslide. It should work fine though.