Using the 'NPC Classes As Background' Optional Rule

If you use this rule, PCs get an extra number of skill points equal to one half the usual amount for a 1st level character of that class. So half of (2+Int mod)*4 for a commoner. These can be spent only on skill groups of the NPC class, or on the specific skills listed on page 26.

These skill points are additional to those the character would normally get at 1st level for their PC class. They do not require you to give up a trait, or skill points in your PC class: they are an optional bonus that some GMs may choose to give. (Obviously, choosing to have been an expert or aristocrat is usually going to be far more appealing than a commoner or warrior)

Basically, the intent of the option is to allow a character to have a background like "blacksmith's apprentice" but not have to spend skill points from their PC class to get ranks in Craft (metal tools).

HPs and Level

The NPC classes get bonus HP for Con based on their Hit Dice, not their level.

Mixing PC Classes and NPC Classes

See Multiclassing for details.