Hello, and welcome to my guide featuringYorick.
I'll tell u how ur combo should be, how u should play on lane, and Pros/ Cons. I'll also tell u why i've chosen the build as i have.

I buyMeki Pendant as first item, cause it gives u the mana regenation u need for ur Omen of Famine. If u got mana, u can get health by using Omen of Famine. That way u can hold ur lane longer before recalling. Tear of Goddess needs to be bought early if u're going for Manamune, cause that way u stack up mana early, and u're getting capped (1000 mana) earlier than u would if u bought it late (ofcourse). I buy Mercury for the tenacity and the magic resist. Some would buy Bloodthirster instead of Spirit Visage. I believe that's wrong, since ur Omen of Famine gives u health back, and Spirit Visage increases the health regained by 15%! Sheen/Trinity force is bought cause of the stack of with Omen of War. And Trinity Force gives u many things included attacks having a chance to slow on auto attack. Warmog's Armor gives u the health for u to get a lot more tanky. And Atma's Impaler is cause of all the health u gain from ur build.

Pros:
- Great lane sustain
- Has ranged spells that givesYorick health
- He is very fast when he uses Omen of War
- Can take both the jungler and his lane at once

Cons:
- Yorick has a weakness for ignite (since his Omen of Famine is the most important spell)
- If Yorick doesn't get neither farm or kills, he's doomed
- Pushes much with ur ghostes.
- Bad at stealing drake/baron since he hasn't got a spell that deals a high dmg output

Yorick is strong as soon as he gets in lvl 3. U have to play defensive through the most of the start. If the enemy champion gets close, u just slow him with ur Omen of Pestilence and after that, u use ur Omen of Famine. These 2 spells are ranged, and therefor good for harrassing. If u get in a melee fight with the enemy champion, u use the "melee combo". The "melee combo" is Omen of Famine, and Omen of War (Omen of Pestilence is only when ur other "omen" spells are on cd.)
Their jungler is always a problem when they come to gank. So if the enemy on ur lane is on 50%- hp, then u should kill him and kite the jungle around, with ur Omen of Pestilence and Omen of Famine. When the jungler is at low health (probably 30%- hp), then u should turn around and use ur "melee combo".

Yorick has 2 ranged offensive spells. U should spare as much mana as possible for the enemy champion, but if the minion with low health is too far away to kill with auto attack, then u should use ur Omen of Famnie on it. U have to last hit the minions, otherwise u'll push way too much.

In ur late game u'll pretty much just use ur "melee combo". If they bring u to low hp, u just use the same method as when the jungler comes to gank (Slowing and using Omen of Famnine to regain health). And remember that ur Omen of War gives u bonus movementspeed, and ur Omen of Pestilence slows their movementspeed.