Choose an appropriate base to hit. Unless the war result is already known (in which case practicing on harder bases than usual is a good idea, or trying out a new attack) try to target a base on which you can contribute to the final war score but avoid hitting a base someone significantly lower level could deal with. Check the attacks remaining. If you're a TH9 and there are three TH8s not three starred but only two TH8 attacks left, cleaning up a TH8 may be the best option. If there are still plenty of TH8 attacks left it would be a waste of their attacks. If you're going for a safe two star, pick a base that is unlikely to end up being three starred, and one which you're confident of two-starring. If you're going for a three star, pick one that you should three star about 50% of the time, and make sure the clan doesn't need you to do a safe two star or a cleanup attack instead. See the guide on War Roles.
Have a plan for cleanup. If you're going for a two star, save a few troops for the end to pick off exposed buildings for the 50%. If you're going for a three star, make sure things like builder huts in the corners don't catch you out.