| Chapmage |
Adding Prestige Classes works like multiclassing.
You can take them when you meet the requirements.
For example, in order to take a level of Battle Herald you must first have the following;
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Base Attack Bonus: +4.
Special: Challenge and inspire courage class features.
Skills: Diplomacy 5 ranks, Intimidate 5 ranks, Perform (oratory) 5 ranks, Profession (soldier) 2 ranks.
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After you meet those requirements, you can add a level of Battle Herald the next time you level up.
Let's say that when you gained enough experience to become level 8, you met all the requirements, so you take a level of Battle Herald (thus becomeing a Core Class:7/Battle Herald:1 character). Then when you've gained enough experience for level 9, you could choose to add another level of Battle Herald, or you could opt to take another level of your core class (at which point you'd be Core Class: 8/Battle Herald: 1)
Hope this helps.