Turn the audio interface and keyboard off. It can be dangerous to make connections between equipment while powered.
Locate the analog mixer's "Main Out." Its location will vary depending on the mixer being used, but it will usually be located on the back panel or somewhere near the top right-hand corner of the mixing board. Labels will also vary, but "Main Out" is common, as is "Audio Out." The connection port will most likely be a standard 1/4-inch audio cable port. Consult the mixer's documentation for details.
Connect one end of the required cable type to the mixer's main out port. Connect the other end of the cable to an available audio input port on your audio interface.
Connect an instrument or microphone to an available input port on the mixer.
Turn the audio interface on, then the mixer. They should always be turned on in that order to avoid feedback.
Open your preferred audio-recording application and configure its preferences to receive audio from the audio interface. Consult the application's documentation for details.
Speak into the microphone or play the instrument connected to the analog mixer. You will hear it play through the audio interface. Consult the mixer's documentation for details on channel functions and other elements of use.