When you fall below -(Constitution+level) hitpoints you die.

When you fall to 0 to fewer, but at or above -(Constitution+level) hitpoints you go unconscious.

Further if you are below 0 hp you become dying.

When dying, you need to make a Fortitude save at DC 10+Hitpoints below zero each round or take one more HP damage.

If you succeed in three consecutive saves you stabilize.

When at below 1 HP but stabilized you are still unconscious but take no further damage. Though of course environmental conditions, lack of food and water or such might damage you as normal.

You heal 1+Con bonus HP in a day, this happens when you wake up after a full rest.

If you do not get enough sleep, you only heal half(round up).

If you do not get enough food and water, you only heal half(round up). If both, round at end and round 1/4 down, higher fractions up, minimum 1.

If you rest the full day, and are at above zero HP, you heal a second time in a day at the normal going to sleep time.

Basic first aid is a DC 10 task taking 4 rounds. Doing it in one round is a DC 15 task.

Proper First aid heals 1 hitpoint, each full 10 you beat the DC by it heals one additional point. It can only be done once until you are damaged again.

If the patient is dying, success by 5 stabilizes the patient, lower success counts as one successful stailization save for them.

A healer spending 1 hour in a day on a patient and using one set of supplies rolls DC 15 healing check.

Success allows one additional healing. Each full 10 success by allows one more.

Long term care in a proper environment with clean spaces and such allows taking 10.

Modifiers:

Hospitals are counted as taking 10 to heal at skill 15 normally for long term care as above. Thus normally in such you heal 4 times in a day if at positive HP.

Smaller clinics and such might have lower skill.

Some special high cost hospitals may have higher effective skill or maybe even count as taking 20 for a constant around the clock active care by multiple people.