Your first option is starting from scratch, just like a normal translation app. As I mentioned above, Fluently is ultimately just a text-editor. So you can go ahead and add headings, fonts, font sizes, and all that fun stuff. But as an added advantage, you can translate to an arsenal of different languages on the spot.
The example above is text translation from English to Spanish, based on text I entered. Option two (2) is uploading a document you already have on your computer, and going through the same process. Fluently automatically detects the language and lets you choose the end-translation.
Note that at the bottom of the image, you even have a word/character count, and file size if you choose to export! Which brings me to the next feature: exporting.
This isn't revolutionary, but it is convenient. This web-app lets you export your newly translated documents into a variety of formats (.doc, .pdf, and so on).