Whenever a PC accumulates 1000 XP, they gain a level. As a guideline, PCs receive 50 XP for low-risk accomplishments, 100 XP for moderate-risk accomplishments, and 200 XP for high-risk accomplishments. The referee should freely notify the PCs of how much XP different objectives are worth when asked.
Designer's Note: This is the way I run advancement because it’s simple and easy to understand. Of course, swapping in milestone advancement, session advancement, or an XP-for-coin system works perfectly well. If using XP-for-gold or silver, note that a gold piece is 100 copper, and a silver piece is 10 copper.
When a PC gains a level, they roll a number of d8s equal to their new level to find their new HP maximum. If the result is less than their previous maximum, their maximum HP increases by 1. They also raise the defense and bonus scores of 3 different abilities of their choice by 1 point. Abilities may never be raised higher than 20/+10.
Designer's Note: You can also raise abilities randomly if you want. My preferred method is to roll a d20 for each ability, in any order, raising that ability by 1 if the roll is less than that ability’s defense. Keep cycling through the abilities, stopping when three abilities have advanced, and skipping any abilities that have maxed out. In this method, natural talents will tend to advance faster than weaknesses, which makes PCs more varied and specialized.