Because this will be the first guide I've ever published, I don't plan on being elaborate or as fancy as the other quality guides. I will tell you what items to get and what skills to learn first though.

Icarus

Skill Order
1. Icarus Dive
2. Sun Ray
3. Icarus Dive
4. Sun Ray
5. Icarus Dive
6. Supernova
7. Icarus Dive
8. Sun Ray
9. Sun Ray
10.Fire Spirits
11.Super Nova
12.Fire Spirits
13.Fire Spirits
14.Fire Spirits
15.Stats
16.Supernova
17+Stats

Justification
I agree with maxing out Sun Ray ASAP, but not at level 7. Icarus Dive is useful for escaping early ganks and can be used to prevent a first blood by utilizing its main usage, Disarm. Disarm causes all affected units to be unable to attack, or use any skills for a brief amount of time. Dive can also be used for last hitting an escaping hero after using Sun Ray to drain their health. Also, maxing out Sun Ray early and spamming it will drain your mana as fast as Storm Spirit's Lightning Ball.

Why not max Sun Ray at level 7?
Mainly because of its damage output at low levels. When up against low hp units like traxex, that start with a little under 500 hit points, the +5% factor does not help out. Icarus Dive will probably deal more damage, and affect a greater area during team fights (not to mention disabling a large oval AoE!).

Sun Ray's main usage when laning is the slow. Most people when under the slow will attempt to flee. Sun Ray is like a glorified Frost Nova, where you can slow the enemy hero down while your ally takes pot shots, or even takes the last hit. Late game, the +5% damage due to hp will be godlike to heroes that have a large amount of hp, like someone with a HoT, or even tank heros like Centaur. Because of this, Sun Ray is maxed Second, during Mid Game when the heroes are a little more beefy.

In most guides, Fire Spirits is taken early. Why is it taken late in this guide?
Fire Spirits is an amazing pusher in most guides. The 35 cooldown however, is not very appealing. In this support guide, you will use Fire Spirits around level 12~14 and after for primarily healing your team mates. Combined with Arcane Boots, you can be a movable burst restoration point, healing your allies up to 400 HP and 135 Mana every 40 seconds. Fire Spirits can be called at the beginning of team fights and used at targeted heroes to heal them while the surrounding enemies take the same damage, possibly winning the team fight. Its still used primarily for healing though.

Items
Icarus is a fickle hero. Consuming his own health to use his main skills, and depleting a percentage of his mana per second to channel one of them drains both substats rather quickly, so what items could you possibly want to get?

Starting Items
4 x Ironwood Branches
6 x Tangos
2 x Clarity Potions

if you calculate the costs, you can get this set even if you -random'ed a bad character and decide to repick Icarus. In fact, if you want to, you can -random and give away the hero you obtained, possibly a favorable pick, have the other random to save 250 gold, and continue passing heroes until 40 seconds remain til creep spawn. At that time, you should repick and get the above items and head for either Top or Bottom. If possible, do not go mid, as your role as support is near zero.

Target Items
Arcane Boots - > (Optional: change to Boots of Travel)
Bloodstone (You can use the Energy Booster from Arcane Boots to complete the Bloodstone)
HoT (for the regenerative bonus to replete your health)
Shiva's Guard / Assault Cuirass (To supplement your armor needs)

Any two items to fit the needs of the game.

Optional!
You can use the starting branches to create Mekanism for burst heal and to increase your armor to an acceptable level. Its best used to fortify your team during a team fight, or to buff your creeps to take a tower down.

Knowing the basic outline is the beginning, learning the hero comes through several games. The best teacher is experience.