Study the music. DJs have to be intimately familiar with the rhythm and tempo of songs before they can blend them properly.
Start the a capella singing of the first song on the correct beat of the second song's instrumental part. The singing may start in the middle of a musical bar and this has to be taken into account to blend them properly.
Scratch the song of the a capella song on the turntable to give it an introduction into the beat of the second song. Scratching the song's vocal part gives the DJ the chance to drop the vocal into the right spot in the instrumental song's rhythm.
Blend the two songs together several times to get the proper feel for the mix. DJs must demonstrate their expertise to listeners by never letting the beat "drop" or miss. Blending two songs together into a unique mix is the signature move of a good DJ.