Load your first song. I recommend using the Left Deck and make sure the master is volume is on there is some trim and there is a volume slider is up and the crossfader is on the left
Find another song it should be in the same key.
Match the BPM (it can be the same or a fraction). use the tempo slider to match the number on the screen there should be 2 circles in rekordbox. I will assume that the first song is on the
Left Deck so the Left Decks BPM will be on the circle on the Left. use the tempo fader and move it up to lower the BPM and move it down to make the BPM Higher
Move the crossfader to the middle and make sure the right deck volume is up and when the left deck gets to the outro press play on the right deck.
make sure beat is matched by making sure the marker with red on the ends of it are matched on both tracks use the Jogwheel to nudge it forward or back