There are lots of ways to do this, but it’s specific to what genre of music you are mixing. Hip hop or top40 usually it’s just a quick cut on the end 32 beat of the first song and on the 1 of the 2nd song. Another thing u can do is use loops, create a 16-32 beat loop before the vocals of your 2nd track to mix in, then turn the loop off just as the vocals of the first song ends and sw in the vocals of the 2nd track.
There’s lots of crafty things u can do with effects, using an echo fade of the 1st song as u bring in the 2nd song works rather well in this situation, or a filter sweep. I don’t advise using effects for transitions until u have properly done transitions using mixing techniques though