I've seen plenty of fail healers around both on Fiesta and on Terra. Thank goodness that I'm not one of them! So for those clerics that often are the target of people making comments like "ur so bad at healing" "fail cleric", maybe it's high time for you to look at my guide!
So let's start with the basic. Your first heal is at level 3, Heal.
This is your basic heal. I took this picture on Terra, so there are no empowerments on the amount healed, while there is on Fiesta. I STRONGLY recommend that you put full points into amount and cooldown, because it's your most frequently used skill.
The next type of heal is Restore, acquired at level 25, which I'm going to put it together with Invigorate, since the only difference is that Restore is not kiteable and it is only for 1 person, while Invigorate is kiteable and can affect an area.
A pretty handy skill, and it does not really need any points put into it, since its cooldown is pretty fast, and it heals well enough. I use it mainly for people who don't get hit alot, or those who get hit too much and I need a break from healing them. It is one of those skills that I use pretty frequently.
Next skill is Invincible. While not exactly a healing skill, it is a powerful skill and one that I recommend to use. Alot.
It is acquired at level 20. This skill makes people take 0 hits for 10 seconds if not empowered, so I mainly use this skill for those who are super squishy, or when I need to revive someone while the tank is taking hits. On Terra, unlike Fiesta, Revive is not a kiteable skill, and there is also a cast time of 2 seconds. The trick to using this efficiently is to not use it all the time, but when it is suitable, like when you need a small break from healing, or there are too many people who need heals. Another skill that I strongly recommend to put points into duration and cooldown.
Next handy skill is Rejuvenate, which is acquired at level 41. Pretty much my second most used skill, it heals a bit more than Heal, but it has a longer cooldown than Heal. If I could, I would add more power to the amount healed, but on Terra, it isn't really possible.
I use it together with Heal, for example, when I need to heal 3 people at a time, I would use Heal first, then Rejuvenate on another person while waiting for Heal cooldown, and then use Heal again on the 3rd person, because it will be ready to use by then. Instant area heal without even using it. It is also handy to heal a large amount. I just use Rejuvenate after Heal.
At level 60, you will get you first area heal! Recover heals a decent amount of health points for party members around the cleric, with a rather annoyingly long cooldown.
I use this skill quite a bit, and now I realize that the 30 seconds cooldown is just not enough for me. I should have put points into cooldown for it earlier -.-
Last skill is Awaken, which is acquired at level 85. It is pretty similar to Recover, but with a shorter cooldown and a smaller amount of health points healed.
Very nice skill, but the only downside is that since the skill itself is so bright, it attracts about as much aggro as Mock would, which isn't handy because you yourself will get hit as well with the others. It is handy for tanking though, I use it alot when I tank in Dragon's Tomb on Fiesta.
Okay, so the basics are sorted out, now HOW do you heal?
Everyone has a particular healing style, and so do I. I always, ALWAYS, keep an eye on the party list, so that I can see who gets hit fast. One important trick you need to know how to use is which heals are for what key. On Terra, I'm using pretty much the same keyboard arrangement on Fiesta, so I know exactly which keys I press will have what skills. Spam Heal as much as you could if the person is being hit, but don't let your eyes stray off the party list, because others may be hit as well. That's how alot of people die in parties, because the cleric wasn't watching their HP bar. So they blame the cleric. Which results in alot of grumbling. Then you get the idea.
I would suggest you experiment with the keyboard to see which skills you will be using frequently, which you won't (for me that doesn't apply), and put them in the quick access bars. Knowing where things are and being comfortable is extremely important, so I would emphasize again, and again, to KNOW WHERE YOUR SKILLS ARE! Personally, I put my basic Heal at 4, because my index finger is one of my most nimble fingers. I put some other frequently used skills, such as Rejuvenate and Awaken, at - and = respectively, because I can reach them with my right hand easily. Also, if you get hit, DON'T use Heal on yourself! There will be a time frame and there are others who may still be getting hit. Use HP stones. They are there for a reason. If things go from bad to worse, use HP potions as well. Who says clerics don't need them?
All in all, develop your own style of playing. What I say may not be true for you, so don't be afraid of experimenting! You might just become a super healer next time. Happy healing!