Riven Mid Early Game
Pre 6 Riven Mid is seriously strong. The early BF sword or brutalizer allows for tons of damage. Poking is key as riven because at some points your enemy will be low enough to all in with ignite which will give huge lane dominance, which is why Riven is such a snowball champion. Post 6 depending on who you face it's gg for your lane. If you're facing anyone like Lux, TF, Ahri, Viktor, or even Diana, it is a good call to all in them everytime because you can dodge skillshots as you move in with your first R pressed so you have the not broken blade which really is broken. When you have a lead ALWAYS roam bottom. Let's say you're 1/0 and you're level 7 with ult. If your enemy realizes you're too strong to trade poke with and they play passive it's an easy way to keep the gold coming. Simply press you're ult when they are pushed a little and all in the ADC or both, even 2v1 you can come out on top.
Mid Game Mid Riven
Riven's strength in the early game can lead to whether she is successful or not successful in the mid game. What to focus on in the mid game is really having lane awareness as well as roam potential. Keeping mobile and attacking other lanes is a huge bonus for characters like Riven because of her gap close so be sure to utilize it.
Late Game Mid Riven
At this point you are strong. It is time to focus the squishies effectively meaning dealing damage but not taking much. When team fights break out it is best to have flash ready because it can be a huge engage tool or disengage tool. Engage: R, Flash, W, Q AA combo,Windslash (2nd R) Disengage: R, Q AA combo, W, Q aa, windslash, dash away, flash out. Late game is the time to make plays so the engage flash stun combination allows that gap close for your team to hop on the squishies and eliminate the carries.