Connecting speakers to your guitar amp can be tricky because you must account for the limitations of the amplifier and the speaker cabinet configuration. Each amplifier is able to handle only a certain number of watts at a particular ohm load, which is the resistance the amplifier gets from the speakers. By matching your speaker cabinets with your amp's capabilities, you can get the best sound from your amp with maximum efficiency.
Locate the speaker jacks on the back of your amplifier. It is important to read the information near these jacks because they will give you the minimum ohm load and wattage amounts for the amplifier contained within the guitar amp. The jacks also determine what kind of speaker cables you can use, but many use standard quarter-inch jacks.
Plug one of the speaker cables into the back of the amp. If it is a tube amp, plug in a speaker before turning the amp on. If you turn it on with no speaker plugged in, you can damage the amplifier. You may also need to switch the impedance of the amp to match the cabinet you are using. Some older amps have different speaker outputs for a different impedance, so plug into the one that matches your speaker cabinet.
Plug the other end of the speaker cable into the speaker cabinet. Some speaker cabinets have switchable impedance switches as well, so match that up before plugging in the cable to assure that your amp "sees" the correct impedance. An impedance mismatch can result in overheating on the amp side and permanent damage to your amp. Solid state amps are much more forgiving.
Plug another cable into another cabinet if using a multiple cabinet setup. Many guitarists use stereo effects and need a more than one cabinet to get the proper sound. Others need the extra speakers to monitor themselves on stage. You can use the other speaker jack to power a secondary cabinet. Be aware that adding a cabinet changes the ohm load on the cabinet, so check the manual for your amp for the correct configuration and calculations to prevent amp damage.
Plug another cable into the back of the first cabinet being used. By chaining them together in this fashion, you may be able to use more cabinets as the ohm load will go up. Overheating is only a danger when the ohms are too low. Plug the other end into the second cabinet. Using this method, you may be able to use four cabinets with only two speaker jacks on your amp.