Alrighty, this guide is supposed to be rather quick and concise since I haven't tried writing a guide in that format yet and my guess is that the majority of people can't be arsed reading a huge wall of text anyways.

Even shorter summarization for super stressed people
Shoot people in the face using Parrrley. Watch in awe as their HP drops by a ton, high five yourself while waiting for Parrrley to be cooled off for yet another shot.

Pros
- Gives the maximum amount of armor penetration
- Together with the masteries and item build, will give max cooldown reduction at level 18
- Takes care of Gangplank's mana issue without crippling the item build

Cons
- Low survivability both early and late game (possible to switch out runes for magic pen, armor or health if need be, this will cripple your damage however)
- Low pre-Sheen damage (possible to get damage runes but I wouldn't recommend it)
- No crit runes (you never want to use crit runes on Gangplank, they're too unreliable and makes for random gameplay)

With this build it's possible to go a solo lane (not against two people though as you'd get crushed) or duo. Paired up with a support hero like Taric you'll absolutely devastate your lane.

Before you've got your Sheen
Don't even try to harrass the enemy champion. Last hit as much as possible using Parrrley and stay on the lane until you've got your Sheen. Your goal should be to have enough cash for a Sheen, a pair of Boots and a few Health Potions. Teleport back to base when you've got approximately 1250 gold to buy these things.

Hurrah, I've got a Sheen!
Sweet, now you should try to use your Parrrley on the enemy champion who has the least amount of armor and HP (i.e. a "squishy") everytime it's ready. What Sheen does is explained in the item section. Continue to farm and harrass, never run into melee range and fight unless you're sure you'll come out ahead. The zoning using Parrrley is more than enough.

- Be wary of enemy creeps, they deal more damage to you than you might think.
- Try to only use your mana for Parrrley for longer lane longevity
- Don't be too quick to use Remove Scurvy as it's your personal cleanse spell. You can use it to regenerate some HP but be sure you won't be getting snared or anything like it before it has cooled off.
- Keep an eye on how the other lanes are faring, it's possible for you to assist from afar using Cannon Barrage

Do
- Poke a lot using Parrrley
- Use Raise Morale before and during the fight (spam it!)
- Use Cannon Barrage during the fight to provide crowd control and maximum area of effect damage
- Go for the enemy squishies
- Be quick to get out of enemy fire if you're getting heavily focused
- Be very careful going melee in team fights, you'll die incredibly quickly if you've pissed them off earlier and they want your head

Do NOT
- Initiate using Cannon Barrage, the damage and slow before a fight is often not worth it. Let your tank or a champion like Ashe initiate with her ultimate
- Try to tank the enemy team. You may be a melee champion but you'll lack in HP, armor and magic resistance with this build
- Run all the way back home when getting out of a team fight, you can still help out from afar using Parrrley

Very early game

This will be your starting item. The idea is to get a Sheen as soon as possible so we can either get a mana crystal or an ability power tome. Since we want longer lane longevity we'll obviously take the mana crystal.

You'll want to rush this as soon as possible. It activates upon using an ability and Parrrley applies any weapon proc. What this means is that the damage you receive from Sheen will immediately be applied to your Parrrley, pretty neat huh? As early as level 5 you can Parrrley people for ~200 damage which is pretty brutal.

Upgrading into Sheen and buying a pair of boots cost 1210 gold

Early Game

Since crit rating works on Parrrley as well we'll want to pick up some on that while increasing our base damage. As I stated earlier, this build is meant to be solid and not based on risky critical strike chance.

and fits the crit chance purposes whilst being useful for building into better items later. The Avarice Blade will supply us with plenty of gold and the cloak will be used to get an Infinity Edge as soon as possible.

An Avarice Blade and a cloak costs 1580 gold

Mid game

An awesome item since it boosts our attack damage and critical strike damage.

It's time to upgrade your boots. Ionian Boots of Lucidity will allow for more Parrrley spam but if they have a team with a lot of stuns and snares, consider getting Mercury's Treads. It's more important to stay alive than to shave off a few tenths of a second of your cooldowns.

This is to increase the Sheen-proc damage, increase our critical strike chance and boost our health. This item is a must have. It's also possible for the Phage built into it to proc which is a nice bonus.

Late game

Gives more HP and magic resistance as well as the spell block, buy this for survivability

Damage, crit rate and the much sought after armor penetration. We already prepared for this back when we bought our Avarice Blade.

More armor penetration and damage, not much to say here

Well, that's pretty much about it. This wasn't meant to be an indepth guide but an attempt for a very short but yet informative guide. What it all comes down to is the normal smart play (never overextend, last hit a lot, ward plenty etc) while spamming Parrrley on the enemies for harrassing purposes. Playing Gangplank isn't really rocket science and I have seen so many terrible players out there so I felt like putting this up here instead =D.

Anyhow, good luck with it all and enjoy your 1.2k Parrrley crits.