When your in any kind of fight, Darius's passive (Hemmorage) adds damage to his ult for each stack, reduces crippling strike CD for each stack, and does damage over time.
With that being said, be aware of your stacks of Hemmorage on the target. Max 5 stacks.
When you're fighting, open with decimate and run up with Crippling Strike, this will damage them then slow them down. If they try to run away pop your Apprehend and pull them back to you. Once they're health is low enough and Hemmorage is fully stacked pop your ult. If you get the kill the Ults CD is refreshed for like 12 seconds or something like that... that means you can use the ultimate again and if you get another kill it'll refresh it again. But if you don't get a kill with his ult the CD is started and its about 90 seconds.

Tips---
With the tanky damage build you'll be able to take quite a beating before your dropped, but if they all jump on you at once you probably don't have a chance unless you have some sort of strategy... Or teamates with you:D Everyone plays differently, just find your own style with this build.

---

Goodluck and have fun ganking with Darius