Lissandra is best at ccing one enemy to death or interfering with an enemy team's ability to team fight with her cc. As such you should play her like that. In lane between 1-4, be more passive and only use your abilities to harass, but be careful not to spam them or you will push your wave and become more vulnerable to ganks. Then at level 6, if you have the enemy laner down to around 60% hp, try to all-in them to get up in a kill. If you can't do this because they are playing too passively or are ahead of you through a kill or something like that, then play more passive and farm because Lissandra is very good at farming and wait for a gank because your cc should guarantee a kill. In team fights you should be either: peeling for your adc, ccing their most dangerous threat to death, and/or getting into the middle of their team and getting a 5 man ult to damage and disrupt their entire team.
Additionally, I like to ult myself in team fights because that usually means a lot of missed skill shots the enemy team wastes on me, so I key bound her ult to self cast on my 8 because my thumb is always over it. I would recommend you doing that so you can always reliability self cast your ult in team fights instead of accidentally ulting an enemy that's near you.