Unfortunately the game tutorial does not teach us something so important in a game in which a Game Over ends its journey (see this guide on bug saving files – there it teach you how to backup your save).
Precious Tips on How to Heal
Well I am a beginner in the game (I started playing today), I will leave tips of the ways I found to heal.
Tip 1: Find “???” locations on the map, they may contain healing events.
Tip 2: Look for an inn.
Tip 3: Have a member of your team with some healing skill.
Note: If you don’t know, to activate a skill you need to fulfill the card requirements used in the battle (there is a sequence that is shown above the character, by filling it out you release the use of the skill that turn).