Step 1: Get any type of heal charm. They work better if they have Enhanced Effect on them.

Step 2: Put the heal charm in either your Main or Accessory slot.

Step 3: Hit Q, E or R to switch between your Weapon, Main Charm and Accessory Charm. Switch to wherever you put the heal.

Step 4: Click on whoever you want to heal. There has to be a monster present, you can't heal without actually fighting stuff unless you and someone else enter the PvP arena. (you can heal yourself in there)

Step 5: Save the day!