First of all, this is my first build so what I say and how I say it may not be as in depth or as good as any other builds you find on here. But they are to the point and it works well at the end of it and that's all that matters to me.
Now to all of you, this guide may seem a little bit silly or stupid or possibly even troll-like. But it's not - because it works. The health and mana regen means it's harder for you to be killed or run out of mana after a certain level due to the Shurelya's Reveries. I actually use this myself every time I play Lulu and it helps me to get a positive KDR unlike when I play certain other champions who I play. It also depends on the game your playing to. So you'll have to bare that in mind.

A bit about Lulu's abilities in short;

[*] Lulu has a passive that is no to little use to a support(or suppordle).
[*] Lulu has utility spells and damage spells which actually increase in potential by going a damage dealer.
[*] Lulu's passive greatly increases potential when we use attack speed.
[*] Lulu has abilities that benefit greatly from quickly appearing, and chasing.

Now I'll explain as to why I chose the runes that I did.
[*] Greater Mark of Alacrity - This is because the attack speed that you will gain from it, will add to the damage due to Pix, being rather quick with his attack speed. Which in the end, helps Lulu.
[*] Greater Quintessence of Alacrity - Again, the same reason as above. Attack speed is your friend.
[*] Greater Seal of Resilience - The armor helps as Lulu can be pretty squishy at most points of the game.
[*] Greater Glyph of Shielding - Helps you to stay alive. Simple as really.

Now I chose this mastery, simply because she will need mana regen, some AP, some spell vamps and some extra damage dealt. This is simply the reasonI chose these, but you can easily play around with them and make them feel better for you.

Four Shurelya's Reveries? Yeah, well check out mana/health and their regens then tell me it's not a good idea. It works for me - end up with more kills than deaths (if any at all) and a rod of ages in there for some AP and the rest of it.
Because Lulu is squishy she needs all the help she can get with health/mana and it's regen. These items actually make it really good for her to be a tank.
The mana and health potions at the beginning are there because she is squishy and they can come in handy at any moment in time.

I just thought I'd give you some information on Lulu's abilities just so you know.

[*] Passive Pix. Pix is Lulu's faerie companion, who deals damage each time we decide to auto attack. Pix attacks with magic damage, so bare in mind it will be reduced by magic resist on the enemy. Simply, the passive causes three shot to be thrown when auto-attacking rather than the one that we'd get without him. If you were to place Pix onto an enemy or an ally, with the W or E, when you were to use Q it would cause the enemy/minions to be attacked from two different directions until he returned to you.
[*] Glitterlance, a powerful damaging slowing piercing skill shot that decays over time. It will be fired from both Lulu and Pix. The enemy can be hit by both, but only one will deal damage. Because this ability is piercing, it will go through minions, monsters and champions, so you can hit several champions with it.
[*] Whimsy, which can be used on either a team member or an opponent. When cast on a team member, it increases his/her ability power and grants a 35% movement speed increase. If used on an opponent, it will polymorph the target, changing it into a squirrel, cat or cupcake depending on your skin, rendering it unable to attack and reducing it's base movement speed.
[*] Help, Pix!, which can be used in two ways; the first way is defensive, by using it on a team member it grants a shield and shifts Pix to that team member; the second way of using it is offensive, by using it on an enemy, Pix will shift to that enemy dealing damage. This can be really useful because we can then fire Glitterlance from that enemy, effectively increasing our range and granting us some surprise attacks.
[*] Wild Growth is a single target health increase (it increases the maximum and current health of the targeted ally) which is not effected by abilities like Ignite; it also knocks the opponents surrounding the target in the air and provides a slowing aura for the target. This ability can be quite good in saving allies, but can also be used offensively.

We build up the Q first mainly because you'll probably end up using that more than the other two unless you need to as it will start to do more damage the quicker you build it up. It's as simple as that.
The sequence will make you more powerful from the start of the game if it's done this way.

With this build Lulu is brilliant for team work when it comes to team fights. Why? Because you have an ult to help someone in need, which ends up having a less than a minute cool down unlike other ults flying around LoL. You also have a sheild, you can transform the enemy which helps the team realise "Oh okay let's kill this guy" and they then can't do damage to you for those few seconds they're a cute little bunny. You can repeat this process so easily that you probably won't even have to go back for health if you play it corrently.
Never get in the middle of the team fight, or you'll end up dead and have to wait - and by the time you respawn - oops. They're are no enemies/team players left because they died.