Use one channel of your powered mixer to supply the main speakers for the audience. If you are using two speakers for this purpose, connect these serially. Run a speaker cable from the mixer to the first speaker, then another cable from the first speaker to the second. In this example, use the left channel.
Use the right channel to connect speakers for monitoring, also in serial fashion, from amp to first speaker, first speaker to second. Most speaker cabinets will have two audio connections for this configuration.
Adjust levels for each output channel. Some powered mixers are designed to be used in this manner and output channels will have independent controls. If yours does not, house and stage levels can be balanced with the pan control on each individual mixer strip. As monitors point back toward microphones, they are more likely to feed back at lower volume levels. Panning to the left will provide less signal to the right channel, thus reducing monitor level.