seriously, it's not hard at all. if you have any musical production talent you can do it. it's just time consuming to find the right parts of the song to use and to equalize it all, but once you get that done all you do is match the BPMs and you're set.
and you wanna hear some of my mashups? I don't bother to make mashups because I make my own original music. take any electro song and some biggie acapella and mix them and you're set...or some pop song. hath is actually pretty damn creative with it though which is why I can respect his music but girl talk and e-603 and that shit I can't listen to anymore. gives my already ADHD brain a shorter attention span.