Summarize this article:

The polarity of batteries is what helps them supply current to a device. The plus sign, or +, indicates the positive terminal. On AA, AAA, C, and D batteries, the positive end should be slightly raised. The negative end of the battery should be flat, and it may or may not be marked with a minus, or -, symbol. There should be a plus and minus sign inside the battery compartment. This will tell you which direction the battery needs to go. The negative end may have a spring or a small metal lever. If the polarity isn't marked on your device, you may need to consult the manufacturer's instructions. It's very important to make sure that each battery is properly aligned inside the device. If your battery faces the wrong way, it could cause your device to malfunction, or it could even cause the batteries to leak dangerous corrosive chemicals. The plus sign on the battery should match the plus sign on the device. As you insert the negative end of the battery, you may press down a spring or a lever. By installing the negative end first, the battery will slide into the compartment more easily. You should then be able to easily snap the positive end into place. The positive end of the battery should snap into place with a gentle push. If you have multiple batteries side-by-side, they may alternate directions. This creates a series of current that amplifies the energy produced by the batteries. Make sure each battery faces the direction that's indicated in the battery compartment or the user manual. Some devices that use multiple batteries may continue to function if one battery is installed incorrectly, but you can damage the device or shorten the life of the batteries by doing so.

Summary:
Look for a plus symbol on your battery. Find the positive and negative symbols on your device. Align the symbols on the battery with those on your device. Slide the battery into place with the negative end first. Check the alignment of every battery.