If it has a Line Out socket, just go from there to a spare Line In on your mixer.
If not, don't take the output that goes to the speaker straight into the mixer--you could burn out the input stages. However, there are DI boxes with built in attenuation that knocks the signal back enough to be safe (and gives you an unattenuated output that can go to your guitar cabinet. Something like THIS. (See, I was listening ecc83 who recommended this in another thread!)