Determine the location for the mixer. This may be onstage if a band member will be controlling sound or in the audience -- called "front of house" or FOH -- if there is a sound engineer. Power amps can be placed at the mixer location or near the speakers. Long cable runs in either setup will require balanced cable, usually XLR connectors between the mixer and amp, or Speakon connectors between the amp and speakers.
Connect cables to your selected outputs on the mixer. Intermediate to high price point mixers may have several different ways to connect outputs. Usually, there are two XLR-style output jacks, labeled "Main" or "Left" and "Right," with possibly a third marked "Mono" or "Sub," for connecting a subwoofer system. These are the preferred outputs to use.
Connect the cables from the mixer to the power amp. If a single stereo power amp is being used, direct the left and right mixer outputs to the left and right amp inputs, respectively. If separate power amps are used, route the left and right mixer outputs to each amp or amp bank. Likely the amps can then be bridged for mono operation. See your amplifier's manual for further information.