You didn't mention what camera you're using, so this advice will be pretty general. We might be able to be more specific if you let us know the make and model of camera. Also, feel free to show us examples of your photos, to better help us see what's wrong.
Macro shooting is a fairly specialized type of photography, and can be technically demanding. I'm going to focus on shooting with ambient light; perhaps someone else will have suggestions for flash (do you have an external flash, or just one built in to the camera?). Here are some suggestions for your setup:
- Use a tripod. For reasons we'll get to, your exposure is likely to be long, and the composition and focus will need to be precise. A tripod will keep the camera still during a long exposure, and allow you to compose and focus precisely without your shaky muscles throwing you off.
- Start without extension tubes, to get some practice. See how close your lens can focus by itself, and set up the tripod at about that distance. The rings might not be as large as you'd like, but you can start getting the hang of things. Once you've got this setup under control, you can start adding extension tubes. You may not need all of them. Each tube you add will let the lens focus closer (but it will no longer be able to focus far away). The bigger the tube, the more this effect will be. If you use all the tubes, you may find that it can't focus far enough away to show the whole ring.
- Focus manually. Set up the camera for manual focus. Once the tubes are in place, you may not have any other choice. But in any case, the focus will be very demanding, and you want control over it.
- If your camera has live view, use it. Live view displays the scene on the rear LCD, instead of the optical viewfinder. Extension tubes "steal" light, making the viewfinder display dimmer. Also, you can zoom in on a live view display, to better judge exact focus.
- Shoot in aperture priority mode, and select a small aperture. In macro shooting, the depth of field is very shallow. That means only a thin range, from near to far, will be in focus. We can mitigate this by shooting in aperture priority mode, which lets us pick the lens aperture, and choosing a small aperture (that is, one with a high number). Small apertures increase the depth of field. Try f/16 or f/22. A side effect of the small aperture is that the exposure time will be longer, which is one reason we need the tripod.
- Set the ISO manually, to a low value like 100 or 200. This will also lengthen the exposure, but the result will be less digital noise (that is, a smoother image). Since we're using a tripod and expect a long exposure anyway, the longer exposure isn't a problem.
- Set the white balance manually. If you're shooting under indoor light bulbs, set the white balance to tungsten light. This may fix the yellow cast to your images.
- Use a self-timer setting. This will allow vibrations to die down before the camera takes the picture. If you have a cable release or remote control, use that too, so you don't bump the camera pressing the button.
As you add extension tubes, remember that you're making the lens focus closer, so you'll have to physically move the rings and camera closer together in order to focus on them.
