You, the DM, should have a full understanding of the rules laid out in the Player’s Compendium. The rules, guidelines, and suggestions in the DMs Codex are used in conjunction with the compendium. Those new to table-top RPGs should start reading at the section titled Rules of Thumb as it will answer many questions. If at any point something is not clear enough or you wish it were more detailed then please refer to the Pathfinder System Reference Document (PFSRD) as it has more than enough rules to handle most any situation. Above all your duty as DM is to keep the game fun so please adjust, omit, add, or interpret things as necessary in order to keep the game fun!