Sounds weird, right? It's actually quite simple. A few tips - this technique tends to work better with prime lenses but you could try it with a zoom if you don't have a prime. Also, I've found that it's better to try this when shooting a still subject. I've tried with bugs and flowers outdoors but the camera is just so close to the subject that every little movement is captured. For the flower examples below, I shot indoors and away from any drafts.
To make an impromptu macro, hold your prime lens backwards against the camera body. Be gentle so as not to scratch the lens. Since the lens is not actually attached, you can't focus the way you normally would. Instead, you have to move either closer to or farther from the subject very slowly until the area you want is in focus.
Here's what I mean by flipping the lens backwards:
The red dot (or white spot for some) is what you would normally line up on the camera body to lock the lens into place.
And here are some examples of shots I've taken using this technique:
Sprinkles on a sugar cookie
Carnation (edited to add film grain)
Molly's chew toy
See? It's pretty easy, no? Just handle your lens carefully and you can get some great super-close-up shots!