This is how your final build should look like :


In case your wondering about how to make space for your Randuin's Omen, First you gotta sell your Philosopher's Stone, Buy a Chain Vest and then go for the Randuin's Omen, From there you buy Trinity Force wich will leave you 1 free room that you use for your Guardian Angel. After that (If the game goes on even after your fully built)
you get your elixirs ( Elixir of Agility Elixir of Brilliance Elixir of Fortitude Oracle's Elixir ) And restock them each time they run out.

HEADS UP

I made this build mostly for myself so I wont forget how to build my darius if i wont play him too long, but the least i can do for the people reading this build is give you some tips.
Now, I know this build doesnt seem usual for darius, but take my word and try it out, you wont regret it :)
If you want a detailed guide about how to play Darius, you should enter this link: http://www.mobafire.com/league-of-legends/build/darius-complete-guide-to-separating-head-from-neck-215726

Regarding the items build, I prefer the way I build it rather then the way the link above shows.

!KEY TIPS WILL BE WRITTEN IN WHITE!

Now for the tips:

#1 : Never be too agressive, but at the same time try to take any opportunity for a kill (And I dont mean to jump into 5v1 for a single kill) Be smart about it.

#2 : LAST-HIT minions only! If you dont seem to manage to last hit the minions, you can allways practice it vs. bots.

#3 : Try to zone the enemy minions near your turret, but not under it because scoring last hits under turret is harder.

#4 : If the enemy minions has allready reached your turret, usually in order to last hit You'll need to do the following:

Meele minions: 2 Turret hits + Basic attack will finish him off.

Caster minions: 1 Turret hit + Basic attack will finish him off.

Seige minions: Cant really tell you, In that case you can either try your luck by last hitting when you think is right or target the minion and look at your ad to estimate if your attack will be the last-hit.

#5 : Allways try to Apprehend enemy's into your turret, then auto attack + Crippling Strike to slow them following a Decimate + Ignite will (In most cases) Either grant you a kill or force him to go base. (allways remember that Crippling Strike resets your auto attack timer, wich means you may auto attack + Crippling Strike very quickly)

#6 : Use your Sheen / Trinity Force right! With the right rotation, The burst damage is crazy.

Example : Apprehend , Decimate + Crippling Strike (+ Sheen [ Crippling Strike + Sheen Together for maximum damage]) And a few auto attack if necessery will in most cases get your target to the required hp to finish him off with your ulti ( Noxian Guillotine ).

#7 : When attacking turrets use your Crippling Strike quickly after auto attacking it in order to finish it off faster.

#8: Allways pay attention to your Hemorrhage stacks and the maximum dmg of your Noxian Guillotine, Dont just ulti anyone that seems to have low hp because Noxian Guillotine is weak without your Hemorrhage stacks.

Memorize and follow these tips if you wanna be a monster darius.
You may allways leave a comment to tell me what you think, give me more tips to add, or tell me if things didnt work out for you [ most likely wont happen tho ;) ]