I am not going to go into an extra detailed way on how to play Mordekaiser. I apologize, but I am sure there are people that have already posted it somewhere. However I do know the best way to build Mordekaiser and I have won 80% of my ranked games with him.

P.S. You will be selling the Needlessly Large Rod (I prefer right after the Abyssal Scepter"

Greater Mark of Insight - more damage dealt.

Greater Seal of Vitality - Mordekaiser needs more health but with his shield/ health pot/ and regrowth pendant he doesnt need it early game. So I prefer the Greater Seal of Vitality so that i can have extra health late game.

Greater Glyph of Force - You can also use a Greater Glyph of Potency, whichever you prefer.

Greater Quintessence of Celerity - Most people will think Greater Quintessence of Fortitude is better, however, I use the Greater Seal of Vitality runes for health. The cool-down reduction is very helpful

boots of speed - get this early so that you can run quickly and it helps be aggressive early game.

Needlessly Large Rod - This will help you get high damage really early in the game and will allow you to be able to complete your build with all the kills it will get you.
P.S. You will be selling the Needlessly Large Rod (I prefer right after the Abyssal Scepter"

Sorcerer's Shoes - Help improve AP Mordekaiser.

Force of Nature - Every Mordekaiser build should have this, it is a good all around item for Mordekaiser.

Abyssal Scepter - More Magic resist and more AP.

Will of the Ancients - I have no clue why other Mordekaiser builds do not have this item. When he attacks enemies he gains health and his shield blocks their damage. Since this is a mostly focused AP build, his healing will be enormous.

Rylai's Crystal Scepter - This isn't the most crucial item, feel free to get whatever you need most by this time in the game.

I apologize for not having the most detailed explanations, however, my build consistently provides me with more victories than all the other builds on Mobafire.