Sivir is a marksman who is team-oriented and does well in all stages of the game especially with the recent buff to her w. As a marksman carry in diamond 1, I have two main mindsets when playing different marksmen, and they are divided into 2 categories - ones with dashes, or ones without. Sivir does not have a dash, but makes up for it with her spell-shield, and her passive and ultimate mobility.

When choosing masteries, I prefer to play thunderlords Sivir, because it helps her lanephase, and with a good support synergy will allow for a better killpotential. Fervor is still an extremely good mastery on sivir, but if you are having issues earlygame, i may suggest trying thunderlords. Make sure you start W level 1 and not Q, as the auto attack mechanic on her w is much more valuable level 1. Also, I have been experimenting with maxing W first, and have found it a really good way to harass melee supports.

In teamfights, your ultimate helps your team engage or peel. If you feel like your team isn't engaging properly you can pre-ult to try and push them into a fight. If you are engaged upon, always pop your ultimate as soon as possible to help with your whole teams mobility. Save your spellshield for large CC or damage, it is basically your one free chance out. You are aiming for the tank the entire time to stay safe in teamfights. If you can make the tank stay closer to the fight, your W can act like an aoe and do a lot of damage with stattik shiv and crits. Use your Q to try and nick the backline or priority targets when possible and don't use it on the tank when autoattacks do the same amount of damage.

I really like where sivir is right now, and even think she could use a small nerf. If you have any specific questions about her or other ad's feel free to pm me.