You have your choice of a bellows attachment, extention tubes or a close up filter as far as getting the close up distance needed for macro shots. The close up filters will offer alot of variation in image quality depending on the quality of the filter chosen. Bellows attachments can get pretty pricy. Extension tubes offer good IQ, a fairly reasonable price, and are simple to use. Be aware though depending on the tubes chosen you may be limited to manual focusing. The length of the extension tube, or tube combination is what changes the magnification ratio thus increasing or decreasing subject size, I cant find it offhand but there is a formula for determining magnification ratio tube length gives with specific lens focal lengths. I'm not sure about the combination of tubes and your 24-70 so someone else will have to chime in on that one. Additional flash may be needed as usually on camera flash doesnt work well with a very close subject to lens distance, a solid tripod is a must and alot of patience.
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I have had pretty good results with the Canon 77 Close-up Lens 500D, albeit mainly with the 70-200/2.8 at longer focal lengths. With the 24-70 you will not get much additional magnification, even at 70 mm. I believe you would be much happier with a dedicated macro lens. I got very good results with the excellent Tokina AT X 100 AF PRO D Macro lens. The current price of this lens is about $450.
Zoom lenses are a compromise of lens physics. So the lens performance is not the best at all focal lengths. You should experiment and see which focal length gives you the best result. As noted you can use the Canon close up auxiliary lens.
You should get a sturdy tripod to hold the camera still and a remote release so triggering the shutter does not move the camera. Also use the shutter delay or mirror lock up to prevent or limit the vibration from the mirror movement before the shutter opens.
You could also use a bellows but then you lose all electrical contacts to the lens and you will have no VR and no aperture control.
Another option is Extension Tubes and Kenko has a series that includes electrical contacts so you have VR and aperture control.
You will need to learn a new technique for focusing as the focus ring determines the enlargement and the camera location forward and back adjust the focus.
Because of the above most Macro purposed lenses have a fixed focal length and lens optics optimized for the best result with that focal length.
See the Micro, Macro & Close-up Photography for more reference and instructional materials. There is a lengthy article with film cameras and the equipment will be different or not available for digital cameras but the physics and principals still apply to the image capture.
DOF in Macro work is very limited unless you perform some focus stacking of images.
All the options are described above but knowing you have optimal IQ with D700 and 24-70. If you would like to keep IQ, I hate to say that you might invest to have Nikon Micro lens.
105 2.8 is recommended by many micro shooters, though I have 60 2.8 for since my close up is mostly still life subjects. I had experiment close-up filters... then I ended up this for my trust worthy D700 (now it is replaced to D800E).