The first thing you need to do is set up your mixer.
Put the channel faders for both inputs (CD or Vinyl) to full, and the cross fader to the middle.
Set the gain controls and EQ controls on both channels to the same setting too.
Whether you use vinyl, CD or MP3 - the most important thing you can do is find an exact cue point.
In most instance, this cue point will be a bass drum - and it'll probably be either the very first bass beat of the entire tune, or it'll be the first beat of a phrase in the intro.
(Check out the Better Mixing page for information on Phrases etc).
With vinyl, assuming it's the very first beat of the tune, this is simple enough.
Just put the needle on the record at the very beginning, press play, and when you hear the first bass beat, stop the record, and wind it back - until you hear silence.
Then wind it forward a tiny amount, until you can hear the bass beat begin to play.
Press stop, and leave the needle there.
You now have the needle cued at the very first bass beat CD's are just as simple.
Pop in the CD and press play.
It's going back to the very beginning of the bass beat that's harder.
If you have something like the CDJ1000's, you can just wind the platter back in exactly the same way as you would with a turntable.
When you hear the bass beat, and you know you're at the beginning, press CUE to store it.
(Most other models of CD deck will probably need you to press PLAY to set the cue).
If however, you're using a CD deck that doesn't have vinyl emulation (like the CDJ1000's) and when you pause the tune, it just repeats that same frame of music over and over, use the controls on your CD deck which let you skip back frame by frame (you'll either have dedicated controls, or just press the search buttons quickly in succession to do this) and wait for the juddering, broken CD sound to stop.
Skip forward one more frame to make sure you're right at the beginning of the bass beat - and then store the Cue point (either by pressing CUE, or PLAY - check your manual).