I could never outline unless I already had a good lot of ideas in mind, so that's the first thing I'd suggest.
Then when I did do an outline (I don't use them anymore), I jotted down all the scenes I wanted to take place in the story. Then I put them in order. (You can do this on notecards or the computer, it doesn't matter.)
Then I looked to see where there were holes or weak points in the story where I didn't have anything happening, and I mulled over what should go in those spots. I came up with more events that tied in to the story (i. e., wasn't just filler) and put them in there to patch up the holes.
Then, once all the scenes were complete and in order, I made up my outline by naming each scene (like a short title--it doesn't matter how creative it is since nobody will ever see it) and using a sentence or two to summarize what happens in every one.
I ended up with an outline consisting of scene titles and brief summaries. These fell into place in chapters as I saw fit when I began writing.
That's how I found it most useful to do it. Like I said, I don't do this anymore, and I did it only a few times, so there are perhaps better ways out there. Doesn't hurt to try though.
