Firstly Richard, you'll need to establish whether the loop offered on your unit is a seamless loop, or just an "A-B loop" (non-seamless; theres a slight, but noticeable gap as the loop re-starts)
Lets say its seamless...
Next, you'll have two buttons, one to set the beginning of the loop, the other to set the end of the loop. These might be called Loop in and Loop out, or "A Point" and "B Point", "Loop in" and "Loop out"...we'll call em "A" & "B"
You'll also have some method of breaking the loop, for when you want the loop to stop looping back to A, when it gets to B, and instead just play the rest of the track from B onwards, eg: After you've finished your voice-over etc. Again this button has many names "loop exit", "exit", exit/re-loop" etc. Some loop exits share their button with the button that sets the "A" point.
Now...its a case of counting and music phrasing. This is the element of most tracks, where you can count 1,2,3,4 1,2,3,4...or more interestingly, count up to 8 or 16 etc, before returning to 1.
Essentially, you want to hit "A" (loop start) on the count of 1, and hit "B" (loop end) the next time you'd be saying "1", so instead of it playing the 17th beat, its playing beat 1 again.
Do your voice-over, introduction, mixing, or simply extend any popular record in this way, and hit "loop exit" when you'd like the rest of the song to be played, from after the B point.
It'll take practice, different units offer different latencies eg: you press the button on "1" and it doesnt notice the press until a fraction of a second later. I've noticed this particularly on hardware controllers for laptop software.
Some cd-decks will also let you edit the A & B points live, so you can nudge a slightly off beat A or B point, forward or backward a smiggin, in case you didn't get it spot on, on the first go.
Now, those Cue 1, Cue 2 buttons. Effectively, they give you the ability to jump straight to another part of the track instantly, Some units will allow you to set the Cue points to be in other tracks on the same CD, for instant jumping between songs.
You unit will normally either have a "rec" (record) type button, or a "clear/reset" button to be used in conjunction wirh the Cue1 and Cue2 buttons, so that it knows what you want it to do. eg: while the tracks playing in your headphones press Rec, then Cue1 on the beat (or word) that you want to be able to jump to. Then just press Cue1 on its own, when you want to start playback instantly from your previously defined point.
Happy playing

urm, I mean practicing...