So this is my Mordekaiser guide.
Mordekaiser can be played several ways, this guide will be about killing the enemy adc as fast as possible and using them to win the teamfights/game.

In the laning phase you should aim to outfarm the enemy as it is hard to kill them unless they are complete ******s. Rush your hextech revolver and farm the jungle too. I also like getting the 'Alacrity' enchantment on the boots for the added movespeed which will decrease the amount of time to move from the jungle to lane.
My normal farming route is: Lane > Wraiths > Lane > Enemy Wraiths
Defensive route: Lane > Wraiths > Lane > Wolves
When you have your ultimate look to roam bot and kill the adc, if successful use the ghost to take dragon.

By now you should have the most gold in game. Work with your team to siege towers and take objectives, other than that keep your cs up. Your cs should equal the minutes, for example, 10:00 = 100 cs, 15:00 = 150 cs etc.

Your job as Mordekaiser is to delete the enemy adc, once he's dead just try to stay alive by dealing damage and keeping your shield high. Using your w on your tank would be a good idea unless they're peeling for your adc.

This isn't exactly the most helpful guide but I'm sure someone will make use of it. Thanks for reading and good luck in ranked with Mordekaiser.