Fighting against Darius, as anyone, is a baiting game. You need to bait out his abilities.
Bait his Q by walking towards him a bit or feigning to kill a minion, and turn around. Doing this should make him use his Q on nothing. This gives you a window to trade or CS. If he Q's, he'll also push the wave, which will keep you safe from junglers while making him vulnerable to your jungler.
As Yorick specifically, do this, and focus on last-hitting with Q to dig some graves. Once you have 4, raise them and sick them on Darius. You don't have to commit yourself necessarily. Just let your Mist Walkers claw at him for a while. He'll have to focus on them. If he misses CS, great. If he uses abilities to kill them, then he's burning his Mana and giving you a safe window to farm.
If you manage to get either
OR
, you should be able to duel him post-6. When fighting Darius, use the movement speed from landing E to get really close to him. Like, uncomfortably close. When you actually want to fight him, it's important to stay on the inside radius of his Q, for the lessened damage and no passive stacks.
Before ever trying to engage a fight on him, though, always wait for him to use an ability. Bait his Q or E, or if he last-hits with W. If you ever want to disengage, it's as easy as trapping him in your W.
Sincerely,
Someone who loves fighting against Darius and Riven
