Drop a little fragrance oil into a small jar of baking soda (I used two tiny capfuls). A baby food jar or mason jar would work well. I didn't have one of those, but I had a small old Yankee Candle Jar (this is how I re-used my larger Yankee Candle Jars). Stir slightly with a fork to spread the oil throughout the baking soda. That's it. You've got an easy air freshener!
I have mine next to my computer. I just remove the lid when I'm sitting here, and replace it when I log off. If you used a jar with a metal lid, you could poke holes in it for a more consistent release of the fragrance. I added a little tea light candle (fragrance-free) just so it would look a little prettier, but it's not necessary at all.