The easiest thing to convert is the monsters. All you need to do is look up the same monster in the C&C Monsters and Treasure book! You can mark the pages in the module and your M&T book with color coded sticky notes so that you always know what page to turn to in a hurry and don't have to bother with making notes.
If the monster you want to convert is not in the Monsters and Treasure Book, I would consult an older edition of D&D-- my personal favorite being the Basic D&D Rules Cyclopedia. If you don't have it you can buy the pdf for 5 dollars at rpgnow.com or buy a used one on ebay.
The monsters presented in the RC are 90% compatable with C&C rules. The only thing that you would need to actively convert is armor class and saving throws. To convert D&DRC armor class to C&C, simply subtract the given AC from 19. Remember that subtracting a negative number is the same thing as addition. So a D&D monster listed as AC 5 would be AC 14 in C&C (19-5= 14). A monster with AC -3 would become AC 22 in C&C (19+3 =22). Saves are easier to convert. If the creature saves as a fighter, thief, dwarf, or halfling it is Physical prime. If it saves as an elf, cleric, or wizard it is mental prime. Use the creatures HD as the bonus to all rolls (including saves, attacks, etc). Simple, eh?
If using 1st or 2nd Edition AD&D as a resource, armor class is also easy to manage. Simply subtract the listed AC from 20. So an AC 5 creature would be AC 15 in C&C and an AC -3 character would be AC 23 in C&C. Saves convert the same as in basic D&D.
If you want to convert DIRECTLY from d20 system, use the same Hit Dice and die type as the creature already has, but get rid of the bonus hit points. So a creature listed as 5d10+15 HD in d20 would convert to 5d10 in C&C. You should also take away any bonuses to the damage dice listed. If the bonus listed is higher than the die, just make the attack use the die type, and double it. So a creature listed as doing d4+5 damage, change it to 2d4. If a creature had d6+3 dmg, just convert it to d6. These changes are because hit points and damage are harder to come by in C&C than they are in d20. If a creature's good saves are FORT or REF, it is Physical prime. If it is WILL the creature is mental prime. If they are all good saves, it is physical and mental prime.
