You have a few options when connecting wires to a small, oblong 9-volt battery. The simplest and most secure is the standard 9V battery snap (sometimes called an adaptor, connector or clip). You can buy one or you can save time and money by salvaging the snap from a dead battery. Crocodile clips, so-called because they look like crocodile jaws, are temporary electrical connectors. These will work as well, but the connections will be less secure. The various items are available at most electrical shops.
Examine one of the batteries. Note that the positive terminal (+) is a circular connector, and the somewhat larger negative terminal (-) is hexagonal or octagonal.
Examine the battery snap, noting the plastic shroud that protects the wire connections. The snap has the same connectors as the battery, with the positive and negative connecting together.
Position the snap so that the positive and negative connectors are opposite to those of the fresh battery, then push the snap firmly onto the battery. You may or may not hear a snap.
Crocodile clip connection
Grip the red clip behind the serrated jaws with your thumb and forefinger. Apply inward pressure to open the clip, noting that it has a spring-loaded mechanism for enabling a quick electrical connection.
Grip the fresh battery and attach the black clip to the negative battery terminal.
Attach the red clip to the positive terminal. Do not let the clips touch, or the battery will be short-circuited.
Recyced snap connection
Grip the dead battery firmly and carefully strip the metal cover completely away using the wire cutters, beginning at an end where the edge of the cover folds over.
Cut the red wire where it connects at the base of the battery using the wire cutters. Carefully remove the snap. If the battery has a metallic conductor strip instead of a red wire, snip it off where it connects to the positive pole on the underside of the snap and solder the 7.5 cm (3 inch) red wire in its place.
Solder the black wire to the negative pole on the underside of the snap.
Wait for the connection to cool, then push the snap on to the fresh battery.
Soldering can be dangerous. Do not attempt unless you have previous experience with soldering or someone with experience is available to supervise.