Hey there! I haven't used her in a while, but I used to be a Riven main once upon a time before her nerfs earlier in the season.
Using Riven's fast combo from her Qs is the same mechanics as orb walking for ADCs essentially. It's all about watching the frames of your attack animation and picking out the moment when the damage has gone through and the rest is all just there for flavor motion. Get a good feel for what point during her animation the damage occurs, because everything after that is entirely unneeded! Once the damage from an auto attack has happened, you can use an ability to cancel the rest of her animations and send her into her Q's animation. After that you're set for another auto attack, and this one is going to be boosted due to her passive. Wait for the damage to happen, hit Q to reset her animation, rinse and repeat.
This is a technique that works for most champions in the game, not just Riven. In general most champions will have one or two abilities that can reset their auto attack timers or cancel remaining frames for an attack. Wukong, Shyvana, and Renekton are all great examples.
Wukong you can attack, hit his Q after the damage from the attack, and it effectively resets him for a second auto attack just after.
Shyvana you can attack, Q for the double attack, and it resets her timer for a second. This works amazingly in conjunction with her E's procs and a BoRK by the way.
Renekton you can attack, W for a reset, and attack again. If you have more than 50 Fury, you can extend this combo into AA > W (increased damage and stun) > AA > W (standard W)
Play around with this mechanic! It can greatly increase the amount of damage you deal in a small amount of time to your enemies, and if you're willing to use some mana early in the game can help you with your last hitting with quite a lot of champions.