I main her, here are my build options
Option 1: Lethality
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Good option early against an enemy top laner who is squishy and you can kill over and over
Option 2: Bruiser
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--> and then tank items depending on enemy team
Option 3: Beefy
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--> Tank items
Option 2 is the one I use the most, as option 1 makes you incredibly squishy, while option 3 is only good late game
I take electrocute on options 1 and 2 most of the time, and aftershock for option 3
I commonly start
, except for option 1 where I start 
Is a very good choice, but only against high cc compositions, but if I need MR I usually go 
Get boots either first or second, depending on when you need them,
and
are my most commonly built boots, but
are build when I need to cap out on CDR
That's basically it to building Riven, but none of this will work well if you do not have the mechanics, but practice makes perfect