At the start of the game, you want to look for a lvl 1 cheese on your top lane opponent. Try to get behind them in the lane bush and rush them down with W if they don't notice you. You could also sit in the tri bush if you are on blue side to catch your opponent from leash return.

Early game you should mostly get push prio due to your strong base stats and passive along with decent minion wave clear with Q max. You can push in 3rd wave and scout jg or cheetah recall in a hard matchup. Always play for the bounce back and try to freeze the wave outside your tower. Freezing is strong on Darius as you favour all in fights with ghost and flash to chase the long lane of top lane.
Alternatively you could create a slow push and stack many waves to then crash and dive your opponent with help of your jg. This way you deny many minions and establish a big turret push.

Mid- game you want to get vision control with sweeping lens and control wards to open up to flanks or ganks on mid or bot. You can look for jg invades as you are strong at this point. You can rotate with your team or split push depending on game state. ARAM fights generally lose resources in the long run so you are better off side laning instead and fights at objectives like dragon soul or baron.

If you make it to the late game, your best chance of winning is stopming a team fight and then pushing end as a team with nash or without. Best thing you could do is deny vision and look for picks or flanks. Save your summoner spells for the team fights and never dive alone. Your team needs to soak pressure so you can deal damage and get ult resets. You could always look for flash E to engage but ideally you want your jg/sup to engage first. Darius is suited for front to back fights where you get 5 stacks of passive off your front target and then gap close with flash or ghost to execute the back line quickly and clean up the fight.