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.