This an excellent video from Hot Cams on how to install a cam in the Honda CRF450R. This is also applicable to the CRF450X.
Step One (Cam removal and installation):
http://www.youtube.com/watch?v=Pu6obWSuqns
Step Two (Valve adjustment):
http://www.youtube.com/watch?v=RmpVjNtZ_yY
Here's a link to our shim calculator:
http://crfsonly.com/calculators/crf450- ... m-calc.php
Ken