It's simple, once you know how to beat match!

While one song is playing in deck A, load up deck B with a new song and beat match them.

Starting keeping the beat of the song playing, counting... 1 and 2 and 3 and 4, in your head. To make sure the tracks are playing in time with one another, be sure to press play on the track you plan to mix in on a 1 count of the song playing. Then simply slide your cross fader over to blend in and out of the two songs.