For starters, you need to learn how to properly manage your passive. Early on, Riven's passive causes her to have extremely unexpected burst. Q-AA-Q-AA-Q-AA-W-AA-E away will end up being your main combo, but only attempt to perform this if ahead. If behind, do Q-E-Q-AA-Q-AA-W then walk away. Once you have her passive down, you need to realize that Riven is a champion with which you need to work for a victory. You cannot, as GD thinks, just ability spam.

"Oh, but she has no mana!"-13 second CD on her main damage spell. This is why you get CDR.
"She has an insane Ult!"-Not really, wait it out, and riven will have wasted it. Problem solved. If she's already on top of you you're doing it wrong.
"But she's OP!"-No. Just learn to counter her it really isn;t that hard. Stack armor. Many people make the mistake of still building AD/AP after the enemy riven is fed. No. Stop what you're doing, finish your major damage item, then go tank. Riven cannot touch tanks. Even with last whisper, she still has an incredibly hard time.

"What??!?!!? Isn't that Yasuo's thing?" Yes and no. When I say flow I'm not referring to Yasuo's resource, I'm referring to the feeling you get when you successfully play Riven. You Riven mains know what I'm talking about. Flow is what I call the feeling of getting a perfect combo; landing that 3rd Q and having it perfectly lead into a Wind Slash. the feeling of even getting the kill with Wind Slash. You'll know you're doing it right when you feel on top of the world after landing a successful combo. Because it's great. And its a big factor of how I play Riven. You just need to keep your flow in check; If you're using your abilities while at 3 passive stacks, you're not playing Riven right.

Riven is unique in that she can be played as an Off-Tank, an Assassin, or as a Bruiser. I prefer to play her as an Assassin, so that's what this guide'll focus on. Back onto the main point of this chapter; wall jumping. In all honesty, if you don't have wall jumping mastered all ready, get to work on that. Its a huge part of what makes her such a great assassin. The fact that with good vision control, you can jump *literally* out of nowhere and instantly have CC on them, and be able to then follow up with more CC, you will be dealing tons of damage in an instant, and they wont be able to react. Also, if your Flash is up and they flash over the wall, you didn't burn flash to engage so you can chase if you desire! As well as being used offensively, wall jumping can be used (And to greater effect in my opinion) to escape. That darned Syndra chasing you through the jungle? No problem! Simply use Q 2 times, then E towards the wall to be sure you're facing the right way, and instantly use your 3rd Q! If done correctly, you will leap over the wall, and if done at blue buff wall on the wolf side, you can then flash back over the other part of the wall for a clean juke. Wall jumping is one of Riven's greatest contributors to her high mobility; use it wisely!

this is very important to your assassination attempts, so listen carefully. You can cancel the cast times of your Auto Attacks, W, R, and Hydra if you use them RIGHT AFTER your E/Q. E works best for W/R, Q is better for Hydra(H) and Auto Attacks(AA).Auto Attacks can also lead into a Hydra if timed correctly in between your AA and Q. I will use /'s instead of -'s to indicate an animation cancel when referring to combos. For example, here's a good combo for animation cancelling: E/R-W-AA/Q/AA/H/Q/AA/Q/Ignite/R

As Riven, you may find yourself having some trouble with your assassinations. Generally what happens is people get CC'd mid dash toward their target, and picked off while the cc is on due to her rather squishy nature. A good way to avoid this happening to you would be to use the Wall jump engage I mentioned earlier, or, my preferred method;
Flash-E/R-H-W-Q/AA/Q/AA/Q/AA/R
Now, while this is a great combo for ensuring you take minimal damage from your target whilst killing them, it isn't exactly the most efficient in terms of DPS. If you really want the most damage output, you will need to be a little more careful.
Flash-R-W-AA/Q/AA/Q/AA/Q/AA/H/AA/E/AA-Ignite-R
Now. while this combo is better for overall damage output, sometimes instead of having all that damage, you just need to do less damage faster, in which case try
Flash-E/R-W-Q//AA/Q/AA/Q/AA-Ignite-R

Now there is one more trick you will want to learn if you really want to assassinate targets well, and that is the Low HP Combo. Keep in mind this combo will only work vs Skillshot heavy champions, as well as having a slight effect on a melee champion who misclicks. A simple combo string won't be enough to explain this one, so I will go into further detail here.
When I say the Low HP Combo (Or LHC, as it will be referred to from her on out), I mean the combo to pull off those clutch 200/2000 health riven vs 1500/1500 health Syndra plays you see so often. The trick here; click behind the enemy with every dash. Your E and Q's will gom through champions, so, heres what it looks like, with MC meaning mouseclick behind them, as well as BH meaning behind them.
Flash-E(BH)/R-MC/Q/AA/MC/Q/AA/MC/Q/AA-W/AA-Ignite-R
Master this combo and you are well on your way to being a great Riven player.

Now, if you're having difficulty assassinating your target, you probably have the wrong target. Is it the ADC? No? Is it a squishy champion? No? Stop right there. Riven has nuke potential, yes. HOWEVER, Riven's Nuke potential only really applies to squishy targets. If your target has 4K health and 400 armor, you won't be getting anywhere when it comes to taking down health bars. Try to ensure that your AA's are focus on squishy targets, as your job is to go in and nuke the ADC, possible some other targets if you live out, then have your GA proc while your team is cleaning up, so that you can get away unharmed. Of course, this relies on GA being up, which has the same cooldown as Flash. A good way to get damage off without your GA up is
E/R-Q/AA/Q/AA/H/Q-W-Flash away
This combo is amazing in teamfights for keeping yourself alive, as well as saving your second ult for when you've already flashed out. so as to prevent yourself from hitting lag via the R's animation.

If you wish to play Riven at a high level, all of these mechanics are necessary. If you want to see all the places where you can wall jump for practice, here you go:
https://www.youtube.com/watch?v=dAjXN99Qg_Y