I struggle securing kills

I wanted to learn riven and started a journey. Currently I have like 17k masteries on her. So, I struggle securing kills. Most of the time I can't last hit and take the kills in situations where my team is grouped and we 2v1 or 3v1. The only time I can last hit the kill is when I use my R2 on a low health champion. Most of the games my kill and assist are equal or my assist is higher. Any tips?