For anybody new to photography, or for the odd macro this is an awesome thing to try, it's tricky to master and very time consuming, or at least I seem to end up really in to it. You get some interesting images and results.
If you're a person with shaky hands or clumsy you can buy adapters that'll mount the lens on backwards on the bayonet plate, which makes this significantly easier. Same goes for those with little hands. If you plan to do it a lot then get them, though experiment with lens tilt is quite fun too.
You can read more of my stuff at killerjackalope.com - my newly renovated website.
Step 1: You'll Need
A pair of hands
SLR (I've not had the chance to do this with a mirrorless but I can't see it not working.)
Your choice of lens is the big thing here, thankfully your kit lens is probably a great starting point. They tend to have a max aperture of no wider than 3.5 on most. That's a good thing here - your aperture will be wide open at all times as the electronics aren't in touch.
I'm making use of an old canon 28-70 F3.5-4.5 The standard kit lens does well, I just broke mine some time ago, not that I used it much. I say old, the lens is only two months older than me...
You can use a nifty fifty to good effect with this, but it takes a little practice as the depth of field is so tiny. Using old manual aperture lenses with this is great, sadly I've misplaced the minolta 50mm I'd been playing with...
Step 2: Camera Settings?
In general I tend to point the lens at the thing up close, while it's on to get a meter of it, your settings should sit about one stop over exposed before turning the lens around.
You have no control of aperture, leave it wide open while metering.
Your ISO and your shutter speed are your main controls, if you want you could use flash in a softbox or if you have an external flash handy, off the ceiling or wall/piece of paper are viable options, the entire thing will be in the shadow of the lens if you're using it going straight forward. (Note - a tall flash like a 420EZ with the zoom at its widest will work pointing forward, however newer flashes often don't have manual zoom control)
Step 3: Focusing and Zooming
Your magnification level is now backwards.
The higher the focal length the weaker the magnification.
At first it'll probably be tricky, you'll constantly be surprised by just how close to an object you can (have) to get.
Below is the broken tip of a drum stick - 22mm reversed at F4.0 - barely intelligible...
Step 4: Results
The eye one was both lucky and unlucky - I was going for focus on my eye and got the reflection instead
The bokeh below is far away streetlights with the lens reversed - I wanted a background for something, now used for my desktop
The tines of this fork look surprisingly beat up up close...
Pills just look like pills really...
The drum stick...