Greetings to all Protectors of the Green!
I hope you'll like my guide for a quite nice Ivy Bot, who isn't only pretty helpful for its own team as a support, but also serves as a pretty annoying offensive Champ. If you declare my guide as helpful, please leave a thumbs up and a comment, thank you!

So let's get started!
So the general rule of my guide is: get a lot of will! It won't only longer your Sustainment on the lane and make you able to spam your skills but also increases the effect of your heal and shield and the duration of your blind effect. Being able to spam your skills enables a lot of your heals because of your passive (Healthy Glow). Make sure you're always aware of your 5th in a row which will trigger your heal!

Skill Order
The skill order is always a tricky thing which always varies a lot depending on the game and the teams but in general I would go like this:
[list]

Always start with Root Strike (Q), it will help you initiating so your mate is able to get the kill. You don't need to max it quickly. If you recognize later in the game that you need less cooldown on this skill spend some points on it. Usually I max it last.

On level 2 continue with Blow Kiss (W) and max it quickly. With it you can harass the enemy well in early game and addionally blind him! Blinded enemies deal less damage.

On level 3 pick Thorn Shield (E) which will grant a shield saving your or your friend's ass.

Always spend your points on your ultimate, Deadly Flora (R), as soon as you can. It is pretty usefull in teamfights, slowing your enemy and dealing constantly damage.

For the rest of the skilling order watch at the top of this page. In my opinion it's like: R>W>Passive>E>Q. Try to vary your skill order depending on the progress of the game.

So everyone says "hey Ivy is support, you need to max your supporting skills first"... my answer is NO! The amount of your will influences your heal and shield pretty much. So try to gather a lot of will first (e.g. with Abin Sur's Lantern) before you max your passive and your E. Early Game it's way better to support your fellow lane partner by harassing the enemy or blinding the enemy ADC.
[/list]
Stolen Powers
I prefer Arcane Green Lantern's / Poison Ivy's Healing Wave for ass-saving, lane sustain or dmg/def-boost:
 or 

and Green Arrow's Surveillance Camera in order to spot enemy wards and place your own:


other possibilities are: Psychic Assault, X-Ray Vision or maybe Teleport. In my opinion the rest of the stolen powers aren't that necessary for our role.

Items
Like I said, get Will! Power Damage is nice as well, it shouldn't be ignored. cooldown reduction also helps you spamming your skills enabling your passive. Moreover it's important to get some items which will actually influence your teammates.

I usually start with:
 
The Medusas Mask (3) and Modular Tektite Vial (3). With these items you can sustain on your lane for a long time! You can max Medusas Mask if you want to, but buy the next two items items as soon as possible. Later in the game, just sell Medusas Mask, you won't need it any more. 
Continues with:

 
Abin Sur's Lantern (2) and Olympus Gambit (2). It is important not to max those items yet!

The Will Battery from the lantern is gathering a lot of will over time by using your skills.  Gather will, maybe upgrade it to Tier 3, but complete the lantern Late game! The effect "Will Force" on Tier 4 is only useful with a lot of will!

Olympus Gambit needs to be on Tier 2 for a long time. The effect "Investment" grants you additional credits over time. On Tier 3 you lose that effect. Try to leave this item on Tier 2 in order to fill your credits!

Essentiel Items:
  
Try to get Eye of Ekron (4) as your next item, additional Will, Power Damage and the effect "Shrapnel" is nice, too
When the Phase of teamfights starts, upgrade your Olympus Gambit to Tier 4!

Other supporting Items you should consider:
  
Amulet of Isis (3) has cooldown reduction and a nice executable skill which boosts your team's movement speed! Metallo's Heart (3) gives you also will, cooldown reduction and attack armor! Enemy ADCs like to pwn away squishy little supports, so you should spend your credits on that.

In late game you definitly need to max your lantern:



Depending on the enemy team, or how much you get focused or wether you want to go offensive as well, the items listed as "optional" at the top are also very helpful

How to play her?
Well that's explained pretty easily, here some tips:
[list]

Try not to steal last hits from your lane partner

Try to iniate with your Q, so that your lane partner picky up the kill

Always watch your team's HP, shield them, try to watch where your passive heal is positioned

Make sure you have a good overview of the map by placing wards and spotting enemy wards

Use all kind of beacons (well that's a general rule ^^)

Be where your team is!
[/list]