DISCLAIMER: This guide is still under construction for season 6, however it is mostly updated.
Also, this guide is based on my play style and what I have been successful with.
Hello and welcome to my first guide ever! As you can see I am focusing on Diana, the Scorn of the Moon, one of "the dash sisters" and imo, one of the scariest junglers to feed.

Hopefully this guide can help you to understand Diana better, and get more comfortable with her and use her in ways that you haven't and get more kills. With that said, lets dive in :)

So for the Runes, you can see that they're mostly either AP, Attack Speed, or CDR runes. The reason being is simple - Diana's passive is a fairly decent chunk of the damage done to monsters in the JG, as well as helpful in finishing off opponents. Her passive utilizes AP and attack speed, as it procs every 3 aa's. The CDR runes are for the combos to be up as often as possible, so that even if you get caught out, not much can stop you. There's also some armor and AD runes in there, and the armor is for some sustain in the early jungling phase so that you have some sustain and don't wind up dying to a camp. The AD also helps with the passive, and just in general.

So for the masteries, they're fairly simple. Due to TLD being borderline broken, I use it, The damage that it does to people, especially squishies, is ridiculous. The biscuits help give more immediate health if someone were to ignite you or if they had red buff. Merciless is great for finishing people off when your combo doesn't do enough damage, which is around mid game when people have enough health to just about get away if you let them. Dangerous game helps going on killing sprees and chains because kills will give you more mana and health. This also provides an incentive to kill. Oppressor goes hand in hand with your smite (unless you buy a different knife upgrade) and your E. Depending on the level of your E, you slow them for upwards of 20%, which means that when you slow them, you'll deal more damage.

So the core items are fairly simple to understand. You get -
Runeglaive
for the AP, the CDR, the mana, and the two passives that it comes with
Nashor's tooth - To proc your passive, for some more AP
Zhonyas - gives good AP, also great for when you overextend or stay. get caught out

Rabadons - I usually get this when I am ahead, to stay ahead by forcing people to deviate from their build and build more immediate MR (which is where the void staff comes in)
Void Staff- Some AP as well as the fact that its the ArmPen for MR
Luden's Echo - All around good item with good AP, and movement speed. The passive procing can also help with clearing waves.

Rylai's - This item is generally good because of the slow, as well as the health and AP that it gives
Archangel's - If you're going to play more aggressively, i would get this item, as it gives you more mana, and AP per amount of mana.
Abyssal - Pretty good item, as it gives you bonus AP, as well as MR, and on top of that it makes enemies have less MR
Hextech - Generally great item, has AD and AP properties so good for usage of AA for the passive, as well as the abilities. This is also a great finisher, because of the active. Can also be used for sustain (what i mainly use it for) with the spell vamp and life steal.

Banshees - Good for the health, and MR. The base regen is good for when the enemy team is in your base, + its passive is pretty dope.

BOOTS

So for the most part, I buy boots of Lucidity on just about everybody for the CDR. If you couldn't tell already, i enjoy using abilities with low cooldown. With that said however, Berserker's Greaves can be a good choice as well for the attack speed. Last, but not least, there are the Boots of Swiftness. I would use these if i really lack mobility, but with Diana, thats not really a problem unless you need to outrun people.

When jungling, you start off with your W, as this does damage, procs TLD if you want an early gank, and when all three bubbles are popped, it provides a shield. This is the main reason you start with the W.

After W, you get Q, so that you can do more damage. This is a skill shot that shoots out in a crescent shape, and can be somewhat difficult to hit at first, but can be easily figured out.

Note that while you do more damage, pre-6 ganks are something i advise against because you most likely dont have enough mobility to lock down a kill like you would after level 6. The last thing you get is your E.

Your E Provides a slow, and pulls people back towards you (Think ori ult). This is EXTREMELY helpful when chasing fast victims down, as it slows, and brings them towards you as well.

The first thing you max is your Q, for the upgraded CDR, then W for shield and Damage, then E for Slow and bringing people towards you.

These are fairly obvious but just in case
Smite - Bcz you're jg
Flash - to get away from death, or to secure a kill
Teleport - If you know for a fact that you are going to be able to stomp, this is an option to move around the map more quickly

Pros

High mobility champ
shreds health
just about forces MR to be built
Scary as **** when you QR from behind a wall with no vision
Can also have utility

Cons

Is somewhat squishy
Q can be hard to use/land
Can easily misuse Ult or run out of mana if not monitered

And that just about wraps up my first guide. I hope you enjoyed it, and I hope it helps to better understand the monster that is Diana. Comments are appreciated so i can improve and make more guides. Thanks :)