Put two records on the turntable. Set the crossfader on the mixer so that Record One is playing through the speakers.
Set the cue controls so that Record One is playing through your headphones.
Switch the cue controls so that Record Two is playing through your headphones before you fade out Record One.
Hold your headphones so that they are only covering one ear. Listen to the two records at once and try to separate them in your head. Adjust the headphone volume levels if necessary.
Fade out Record One and bring in Record Two at a point when the beats match well.