Hey! My IGN is Sir Vahhgs and although I may not be the greatest at this game. I feel as though I have played Graves enough to publish a forever changing guide for him. I shall include videos in my guide as well but those will come later. For now I will stick with a more words on paper approach.

Pros

High damage output using Buckshot
Great mobility and Attack speed using Quickdraw

Has an awesome CC ability as well as limiting the F.O.V of the enemy team with Smoke Screen
Not to mention is ultimate Collateral Damage
And best of all his passive True Grit which boosts your defensive stats. This gives you a higher lane dominance.

Cons

Very squishy
CS and Kill dependent
Low movement speed

Marks

Greater Mark of Desolation x 9 = +15 Armor Pen.

These will boost your Buckshot. Which will be your main source of damage output. Armor Penetration is very important when playing Graves.

Seals

Greater Seal of Attack Speed x 9 = +6.9% Attack Speed

These runes will increase the benefit of your Quickdraw. Significantly increasing your damage output through basic attacks and making good use of your defensive passive True Grit.

Glyphs

Greater Glyph of Critical Chance x 9 = +2.5% Critical Chance.
This will boost the chances of you landing a critical strike. Works well with Quickdraw

Quints

Greater Quintessence of Desolation x 3 = +10 Armor Pen.
Adds more BANG! to your Buckshot and Collateral Damage

Something went wrong with the Masteries Planner. In the Offensive Tree I grab that point in Veteran's Scars . It wouldn't let me.**

In the Offensive Tree I stick with Attack Damage, Armor Pen, Critical Chance, and Lifesteal. Improving damage output via Buckshot, Auto-attacks, and Collateral Damage

In the Defensive Tree I stick with Health, Armor, and Magic Resistance. This makes your E Quickdraw so much stronger and more viable for spamming.

Recommended
Flash
When Flash is paired with Smoke Screen and Quickdraw, He becomes potentially ungankable and can easily gank other lanes.
Ignite
I take Ignite for the soul purpose of securing a kill or taking down carry's faster (diminishing their lifesteal or spell vamp.)

These summoner spells, in my opinion, are the best for Graves

Other Viable Options
Exhaust
Exaust is great for shutting down enemy carries and synergizes well with Quickdraw for that pro escape or pro gank.
Ghost
Ghost is another fine choice. It increases the usage of Graves' Quickdraw and Smoke Screen to escape like a pro or get that PENTA!

I start out with a Doran's Blade. Gives you some health, AD, and Lifesteal. Its a great starting item for the typical AD Carry. But on some occasions if I am feeling confident with my team composition, I start out with Boots and 3 x Health Potion

If you started with a Doran's Blade, before recalling for the first time, make sure you have enough gold to snag Boots Boots 1 (if you didn't start with it) and another Doran's Blade. This will significantly increase your farming ability as well as maintain a certain level of harass against your lane.
Grab Berserker's Greaves

If you started with Boots and your Health Potion, On your first recall, have enough to buy either two Doran's Blade's or a Pickaxe. I would lean more towards the Doran's because it gives you Lifesteal, AD, and Health. Everything you need to dominate your lane.

After that try to maintain your CS above that of the enemy AD carry. You're gonna want to rush Infinity Edge. This will increase your Critical Damage by 50% (200% to 250%). This paired with your Armor Pen and your Quickdraw makes for a very intimidating lane presence.

Once you have obtained your Infinity Edge, I suggest grabbing a Vampiric Scepter which rewards you with 10% lifesteal which is added to the two Doran's Blades. You'll have around 16% lifesteal. Your next goal is to build Bloodthirster. Which is an amazing item on Graves because it grants him the ability to farm up his lifesteal and AD ( Graves is amazing at farming. I'll get to that later.)

Building a Phantom Dancer will benefit your Quickdraw and works well with Infinity Edge.

Its late game now and you have no idea what to build next. Well let me help you. My suggestion would be to assess the game Kill to Death ratios and Tower ratios. If your team is doing well then you can take the hyper carry path and build a Last Whisper and a Black Cleaver. Both items will significantly boost Armor Pen and Black Cleaver will give you attack speed again benefiting your Quickdraw.

If you're getting focused in team fights then I would suggest building some defense with offense with items like Frozen Mallet and Trinity Force.

Enemy is Magic heavy = Build Banshee's Veil
Enemy is Physical heavy = Build Frozen Heart
Gives Armor, Mana, and Cooldown Reduction.
Buckshot SPAM! = "TONS OF DAMAGE!"
Or you could build Guardian Angel which supplies you with Magic Resistance, Armor, and a nifty zombie-like passive ;) Gives you the ability and opportunity to escape with your life or continue to deal "TONS OF DAMAGE"!

Also buy Sight Wards, Vision Wards, or Oracle's Elixirs as needed by you or your fellow teammates. Be attentive to what wards you will need! Twitch, Evelynn, Teemo Shaco, Akali, Wukong, and Vayne

I prioritize Buckshot. It's Graves main source of damage alongside his auto-attacks. I max this first.

I max this second Quickdraw. This allows you to harass your enemy, Escape enemy ganks, or increase you ability to farm.

I max this third Smoke Screen at level 4. This is a great harassing tool as well and it pairs well with Quickdraw for escaping enemy ganks, harassing your lane, ganking the enemy team, or initiating team fights.

I level Collateral Damage whenever possible. Great initiating tool as well as snagging low health enemies at a distance.

I made this the last section of my guide because I feel that Farming is very important for Graves. Same could be said about any carry and for the game as a whole imo. But early game farming is crucial. Try to harass with basic attacks and Buckshot. I am very good at farming and my build is a bit on the expensive side. That's why I am bringing the importance of farming and my guide.

LAST HIT THOSE MINIONS! You could also use Quickdraw and Buckshot to help farm faster. But be careful! If your bushes and your side of the river isn't warded, I would just last hit instead of clearing waves with your abilities. Without Sight Wards or Vision Wards you will be very susceptible to ganks.

Play Video

Graves has a lot of potential to hyper carry a game to screaming victory. He is heavily reliant on kills and Farming. LAST HIT THOSE MINIONS!. Don't be afraid to dive into the fray! With Smoke Screen and Quickdraw, boosts your defensive stats making you more durable while increasing your attack speed. Means more damage output. All this while your Smoke Screen blinds your enemy.

As a player, you must have excellent map awareness because you have the ability to shut their F.O.V down and dash in for the kill(s).

Please feel free to comment with your opinions on what you might change or add. I am always open to constructive criticism, not harassment.