Due to the extremely small depth of field and high magnification, you will need:
- A lens reversal ring (make sure you choose the right thread size) to mount the lens onto your camera backwards
- A tripod to keep the camera stable
Even better if you can also get:
- A Nikon BR-6 adapter
- A Nikon AR-3 shutter cable
- A lens whose physical and focal length don't change when you focus.
- A wireless shutter remote
The BR-6 adapter goes on the back of your lens, and enables you to lock the aperture wide open. With the AR-3 shutter release cable, which screws into the BR-6 adapter, you can adjust the aperture yourself with a button you press down. You can set it to stay wherever you leave it; i.e. an any aperture. Unfortunately there's no aperture scale, though such a scale wouldn't be much use anyway since the f number is different when you mount a lens in reverse. These tools aren't required for an AF lens with an aperture ring that lets you adjust the aperture manually. But they are very handy for lenses without an aperture ring.
Another important consideration is that if you use a lens that changes physical length when you focus, then this can result in having to physically shift the tripod/camera every time you want to change focus.
Photos of Setup
Here's what the lens looks like with the reversal ring screwed in where a filter would normally go, and the BR-6 adapter attached:
At this point, you can lock the aperture either wide open or fully stopped down, but nothing in between. Unfortunately, the BR-6 doesn't provide this capability. Unless....
...you add an AR-3 shutter cable to it. Now you can push down the button to the desired aperture, and it will stay in place. The cable has the added bonus of not disturbing your camera as much when you want to adjust the aperture. The camera is very sensitive to slight movement when shooting macro.
Here's what the camera looks like with the adapted lens mounted:
And here is an example (focus-stacked) image taken with this setup:
Your technique of holding the lens by hand is sometimes known as freelensing. While this can produce tilt-shift effects that look interesting in some situtations, it will probably make macro focussing nigh impossible. If you don't align the optical axis of the lens exactly perpendicular to the camera's sensor, and intersecting the center, then the focal plane will be tilted and shifted.
i.e. here's an example photo I took while freelensing for selective focus on a subject's eyes (although I didn't do a great job):
I have not added blur to this photo after the fact. This is how it was taken. That's caused by a tilted focal plane, something which will only compound on the difficulty of precise focus when taking macro photographs.
When taking macro photographs, the depth of field is extremely small because of the high magnification. So small, in fact, that you may be tempted to use the smallest aperture available and end up with diffraction limited photos as in my case here. There are a couple of solutions:
Take long exposures on higher f numbers (being wary of diffraction), and make sure your camera and subject are perfectly still. This is where a shutter remote comes in as well.
Take a series of photos, identically composed, at different focus settings. Use software after the fact to merge these photos (i.e. Photoshop's Auto-Blending of Layers does a good job).