Plug your guitar amp's power source into the wall, but make sure not to turn it on. Set it on top of the cabinet (cab).
Find the output source on the amp. Look for a jack labeled "Output," "Speaker" or "Headphones."
Plug the guitar cable into the amp's output, and connect the other end of the cable into the back of the cabinet. There are usually two holes on the back of the cab, sometimes they're labeled, sometimes not. Look for a label that says "Input." If you can't find one, plug the cable into the left-side input first.
Plug your guitar into the input of your amp, which should be stacked on top of your cab. Turn on the amp and play your guitar to see if it works. Make sure the volume knobs on both your guitar and your amp are turned up. If it's still not working, turn the amp off, and try connecting the output cable from the amp into the right-side input in the back of the cab, instead of the left-side input.