This is how I do mine. I get the acapella version of a song. I then take a loop of the original song (just 4 measures usually the intro). I put the loop into acid so that I can get the tempo. If you right-click on a loop you can get the actual tempo if it is a perfect loop. Next I put the acapella version into acid and line it up with the loop so that I can be on the basic count. Then I just record stuff from my keyboard onto acid to go with the acapella version. And that's my remix.
I use Acid 2.0 by the way. I didn't really like acid 3.0, it had too much stuff.