When I shoot with natural light, I almost always use the Av mode (Aperture Priority) and Matrix Metering mode. I decide what aperture I want, which is normally F11 (or higher when i am greedy for more DOF and the background is not too distracting), and let the camera's light meter decide the shutter speed. I normally set the ISO to 200. ISO100 is preferred but when it is windy and the subject moves a lot, ISO200 will mean half the exposure time and higher chances of sharper shots.
Take a look at the following shots of a robber fly under back-lighting condition:
Robber fly, no reflector. Note the heavy shadow.
Robber fly, with silver reflector.
Robber fly, with gold reflector
Try to place your reflector at different positions and angles to see how best to throw back some light onto your subject for the much needed fill-in. You will also find out that some times, the reflector will make almost no difference at all.
If the backlighting is not too severe, you can always adjust the EV (Exposure Compensation). In this case, of course, I added about +1 EV. But this will work only if the back-lighting is not too severe. Notice both the subject and the background are brighter?
Of course, you can also spot meter off the subject and get the correct exposure for your subject.
All the images were shot either under complete shade or when the sky was overcast/gloomy, thus no hotspots on the subject. However, if the light is too harsh, you will want use a diffuser to modify the light, as explained in this post.
Please also check out "Tips on shooting with natural light" for more info on other cool gadgets - plamp, camera bean bag, macro focusing rail, etc - that will will come in really handy in your natural light macro photography.
How to be a "reflectorogist"? I was the one holding the reflector (i.e the reflectorogist :D). Derrick was taking natural light shot of a cute Earless Agamid (where the red arrow is pointing) while I held the reflector in place to reflect some light for fill in. Image courtesy of Chow CK.