Play your first track. When you're ready to introduce the second track, select it so that the sound is coming out of the headphones only.
Listen to the beat of the track playing through the speakers and then to the beat of the other track, the one playing through the headphones. Adjust the pitch of the second record until the two records are playing at the same speed.
Match the two downbeats. If you divide a song into series of eight or 16 beats, the first beat of each series is referred to as the "downbeat." To match the two downbeats, nudge the second record either forward or backwards. Matching the downbeats is probably the hardest part of mixing and takes time to master.
Introduce the new song at a quarter of its eventual volume, with barely any low frequencies and only half of its mids and highs. Keep the bass around 30 percent if it's a bassy track, and keep the high knob turned down a bit, too. Sometimes it's alright to leave the mids at a normal level because it's nice to have intro vocals or synths in front while the rest of the track is playing at a low volume.
Listen for your cue. Most dance music tracks have specific parts that are meant to help the DJ mix in a new track. These sections usually feature rhythmic parts only, so there's no main melody and few (if any) vocals.
Decide beforehand how you would like to make the bass transition. Know exactly when you want the new track to be in full swing so you don't get caught playing a meaty part of the track while adjusting the EQ.
Slowly adjust the EQs and raise volume on the incoming tune, while doing the opposite on the tune you're mixing out of. Fade out the old track, bring in the new and get ready to do it all over again.