If they have an option to download in ePub format, that will work. You can download them directly to the Nook Tablet with the built in browser.
Once they are downloaded, you can launch them from the download notice, and get back to them from the recently read option on the top right hand side of the home screen, and you can get to them by going to "Library", "My Files" and "Downloads".
If you are not going to read it in one sitting or are downloading several at once, it may be more convenient to install the "OpenExplorer" Nook app and move the file from the "My Files/Downloads" to "My Files/Books".
If they are not in ePub format, if it was me, I'd probably convert them. Convert to EPUB describes how to do that on your computer using a free program called "Calibre". Once you have Calibre set up, and then plug your Nook Tablet in with a USB cord, its possible to drag and drop a book in another format onto Calibre's "Nook" device and ask Calibre to handle the conversion for you.
If the files are copy protected, Calibre would also need plug-ins to handle that, but fanfic is not likely to be copy protected.
I hate to appear illiterate, as I am not, but please explain fanfiction. Thanks
I use flagfic.com, which allows you to download an epub version of fanfics from specific websites - all you need is the url of the fanfic. You'll have to look through their list to see if the website you use is supported, but I know they support fanfiction.net.
You are going to have to DL a program like calibre if you want to get fic onto your nook. AO3 makes it really freaking easy to DL epubs right to your nook. But the fic would have to be posted there in the first place.
If you want the hassle you can go to your word doc and print it as a PDF. Nooks can read that. It's a pain in the ass. You wouldn't have to dl anything.