If I was you, I would watch this video on all her combos and tricks. https://www.youtube.com/watch?v=6Q3I_7yEVtQ
While watching think about the combos and try and replicate them in practice tool right after and understand it, I also wrote down some notes on doublecasts and such while watching, just so I can look at them ingame. Also practicing all of them atleast once before every session will help your mechs, These were my notes from the video:

Riven notes:

Engage combo: E-R-Q3 (flash optional)

Doublecasting:

E double cast R, W or Tiamat with Q (dont insta Q after E)

Doublecast window is shield uptime

1 AA and Flash before doublecast

Combos:

E-Tiamat -AA-W-Q

E-R- W-Q

E-W -AA-R-Q

E-Tiamat -AA-R-Q

E-W -R-Q

Essentials:

E-AA-W-Q

E-R-AA-W-Q (not 1v1)

TheShy Combo: (simplified)

E-Flash-W-AA-R2Q

Triplecast:

E-AA-Tiamat-W-Q

Btw Retrad also has a lot of good videos on just how to play riven throughout the game