most people just take 4 bar loops from mainstream songs usually from parts of the end of the song where no one is rapping or singing say "YO MIXTAPE SHIT Blah blah" on top of that for an intro, loop those 4 bar loops into 16 bars and then slap the chorus on the 17-25 then repeat the first 2 steps till you are done with the track then use the very outro you looped and close the song out. If you want plain instrumentals just run a limewire search on songs you want and hopefully you'll get lucky and get them. If you dont find the instrumentals just get yourself real handy with something like Cool Edit Pro and you can chop up a beat. Thats about it give or take.