If you own both of the two lenses you mentioned, just keep one on the camera, then hold the other (reversed) in front of it (you can tape the two lenses together if you want).
Doing this you'll get extreme magnification; on the other hand, depth of field will be close to none. You'll probably want to avoid getting most of the image black so
- keep the aperture of the reversed
lens wide open blocking the small
lever in the mount with a piece of
paper or something similar
- if the on-camera lens is the zoom (which seems to be better, at least in my experience),
zoom it to the longest focal.
Switch autofocus off and don't bother with manual focus, you won't need it as moving a millimeter will change what's in focus. As this technique reflects some problems of a huge focal length, it's better to have enough light to use the camera in shutter priority with a short time (e.g. 1/300 s).
You could just reverse ONE lens, buying a reverse ring or making one but not all cameras allow you to shoot w/o a lens on, and/or the meter wouldn't work.