Take a piece of tissue–toilet paper will do–and roll it into a long, thin pencil-like shape, with a sharpish point. Leave a bit of tissue unrolled, so you’ll have something to grab onto when you’ve had enough.
Now take the wad and stick it up your stuffy nostril or nostrils.
If you’ve got a cold, you may notice some inflamed lining at the back of the nostril. Essentially, you’re using the pointed tip of the paper to push the bump over to the side, allowing you to breathe. Imagine how glam I looked going to bed with a stuffy nose and humongous curlers to straighten my hair!