To use the lens for close-up photography, zoom to the max focal length (e.g. 300mm) and run the focus to the minimum focus distance. The minimimum focus distance is fairly close consider this is a 70-300mm zoom lens... but it isn't anywhere near as close as a true/dedicated macro lens.
You can get even closer... there are two ways to do this.
1) You can use a close-up diopter. For that lens you'd want the Canon 500D close-up lens (specifically you want the version with 58mm diameter threads to attach to the front of the 70-300mm lens). It's about $85. This is a two element lens (which is much higher quality than any 3rd party close-up diopter).
2) You can use "extension tubes". An extension tube is (believe or not) exactly what the name implies... it's a hollow tube... and it extends the distance between the lens and camera body. You attach it to the camera (just like a lens) and then you attach the lens to the extension tube instead of the body. This positions the lens farther away from the image plane inside the camera and, as a result, the entire focus range of the camerea is shifted nearer to the camera (closest focusing distance was 1.5 meters without an extension tube, but will drop to just 42 centimeters with the Canon 25mm extension tube attached.) It not only allows the camea to focus at a closer distance, it also enlarges the size of the image.) Extension tubes do not have any glass in them -- they are completely hollow. The body of the tube does have electronic pins to pass signals through so that the camera body and lens will still be able to communicate.
The Canon 25mm extension tube is about $140. Kenko is another popular brand (their tubes come as a "set" of three different lengths).
Whether using extension tubes or close-up diopters, Canon suggests that you should manually focus the lens.
When you get to very close focusing distance, the depth of field can become quite shallow. So shallow that you may not get as much of your subject in focus as you want. Hard-core macro photographers will use (a) a tripod (because after precisely focusing... if your body moves by even a few millimeters it can completely throw the subject out of focus). So you do want to make sure you do not move (and the best way to do that is usually to use a stationary object to hold the camera -- like a tirpod.)
They may also use a technique called "focus stacking". Focus stacking involves taking several pictures. You take a shot, tweak the focus by just a few millimeters and take another shot, and keep repeating until you've got enough shots to stack the subject from front-to-back with all the different focus positions. The images are then merged using computer software (Photoshop will do this, but there are third party products that are dedicated to focus-stacking as well.)
I've done this by wrapping a piece of masking tape around the focus ring on my lens. I then focus to the nearest focus point and use a marker to mark that point (on the tape). Then I focus to the farthest focus point I care about and mark that spot on the tape as well. Now I take a series of photos ... taking each, then slightly adjusting focus a tiny bit at a time, take another photo, and repeat until I've taken enough to get from the nearest to farther points I've marked on that tape.
Harder core macro photographers will sometimes use something called a focus-rail. This is a track which you mount to the tripod and then attach the camera to a carrier that rides on the "rail". The rail has fine adjustments which allow you to move the camera with control. It you run the rail back (farthest point), then focus your lens to the closet focusing point on your subject. Instead of turning the focus ring on the lens... you use the knobs on the focus rail -- which physically moves the camera forward.
Tim Campbell
5D III, 5D IV, 60Da
. Even the manual I've found doesn't really give instructions re: macro, even though the lens itself says "MACRO 1.5m/4.9ft" right on it.
