The first thing you need to know about Karthus is that a bad pusher is a very weak unless the fed, so here you have some advice.

Always be aware of all the parts of the map (This is extremely important, as this depends on you to kill or get assists, and thus fill the mejai)

Never pressure in the middle, stay close to the tower if there is a champion in the jungle, take a chance only if you flash, but do not recommend it.

The team struggles always keep a little away and behind all of it and get right in the middle, even if you die, your area, your ultimate and your Q will allow you to do many deaths.

ANOTHER TIP:
A dying Karthus lot = ****, so even if not kill, attempt mostly NO DIE

The main purpose of Karthus is undoubtedly the reason Mejai.Por this will be our first purchase, and we can begin to fill as soon as our ultimate.

Why boots later? Because as I indicated before, we have no plans to move a lot of tower, since we are a "weak" character.

We can exchange the order on the hourglass and the empty cane.