Really it's just going into a custom and Q minions and autoing.

Also, you want to cancel your auto once you hear the auto attack sound on the minion or see the sword touch the minion. Practice auto attack cancelling first and get the feel for it.

After hundreds of games on Riven, I have become somewhat comfortable with almost all of Riven's combos. The only one that still eludes me is the Q cancel by clicking the ground and then re-clicking the enemy. How would I practice this?

How do I practice the Riven Quick Combo?