Season 4 is the rise of AP burst supports. With the rise of Annie, Leblanc is another great pick. She has great kill potential in soloqueue and can snowball out of control. Very safe with her Distortion + Ult, and can provide near-instant silences with Q+Ult or a snare-lock with her E+Ult. Her base damage is very high, so she does not need damage items until mid/late game.
The flask start is for more sustain in lane. One of Leblanc's greatest weaknesses is getting poked until she cannot trade. However, with flask and a biscuit, she can easily stay healthy and stay relevant. Two sight wards + your trinket and your adc's trinket is also enough vision to stay safe.
Ancient coin is the superior support gold-income item to spelltheif due to its great active and easy income. It also helps you sustain. However, only upgrade to the Talisman of Ascension once teamfights start happening. In lane, your sightstone and DFG is more important. Next, get a Zhonyas / Rabadons / Void Staff / Guardian's Angel. These items are core, but you must make the decision of what to buy first. Zhonya's is an especially great item due to the fact that it can distract the enemy, and give you the time for your w to come back up.
In lane, you can either start W or E. I prefer W 90% of the time, to be safe and to provide instant burst if needed. E provides the slow and snare, but it is risky due to the fact that it is easy to miss. Get the other option at level 2, and start maxing Q from 3 onwards. In the early levels, your aim is to push the wave so that you hit level 2 before the enemy. If you succeed in doing this, you can W in then snare one of them, and your ADC should be able to burst them down with their level advantage. If your team does not hit level 2 first, then ignore this plan. Play passive until level 3, and then Q + W + E them. By using E last, it is easier to hit and provides more time for your ADC to respond to your play and start attacking them when they are slowed. If the enemy support like leona/annie engages, you have two options. 1. Burst down the enemy ADC with your own ADC. 2. Kite them with your chain and back off. Obviously #1 is the better option, but recognize your damage potential with ignite and do not carelessly use your W. If ganked, you also have these two options. LEARN YOUR DAMAGE POTENTIAL, YOU ARE SLIPPERY WITH YOUR W + ULT + FLASH SO BE BALLSY AND GO FOR THE KILLS
By midgame you should have got a sightstone and a fiendish codex / large rod. Maybe a DFG if you got fed. Use your vision (sightstone) to assasinate opponents and help allies with your chains. Use Q to whoever you're chaining to silence them as well. Save W for your escape or to catch up for a kill. You can also catch out enemies from long distances with W + Ult + E + Q . Make sure to use your silence wisely, and don't forget to use your Talisman of Ascension when you complete it. Your maximum damage combo will be DFG + Q + Ult + W + E, don't be afraid to use it on the front line if your team is having trouble.
Leblanc is an underrated support champion, her problems with farming and waveclearing is made null by giving the farm to the ADC and her high base damage makes her lack of items meaningless. I hope you have the same amount of success and fun I did, good luck!