This is a quick (relatively, it’s 18 minutes long) video tutorial on converting an existing single style font to a multiple master/variable compatible font in Glyphs.
It’s not a perfect tutorial, but it should be enough for you to get started if you’re on a mac. As mentioned in the video, this is all possible (and in some ways easier) with Fontlab 6, I’m just still learning the ins and outs of that new version.