You can get greater magnification with extension tubes between the lens and the body, so long as the extension tubes' length does not exceed the focal length (not a problem with 70-210.
The usual MO for macro is to use manual focusing, and set the focus based not on distance but on image size*, then move the camera/lens backwards and forwards until the subject is in focus. Depth of field is inevitably very small, so focus can be hit and miss. One method for hand-held macros (if the hardware permits) is to move closer to the subject and then fire a burst of exposures, continuing to move forwards, in the hope that one of the exposures might be in focus. Of course if the subject is static, then a tripod (actual or improvised) is very useful.
*my Olympus 60mm macro lens has a handy scale marked 1:1, 1:1.3, 1:2 and 1:4 and that's what I use.