How to sue Russia because of the war?

Judging by the decision in this case (https://reyestr.court.gov.ua/Review/117208926), you can file a lawsuit in court, but, most likely, you will be denied a lawsuit. You can file an appeal in case of refusal. In this case, the Cabinet of Ministers filed a withdrawal of the appeal, referring to its groundlessness and lack of evidence. But there is one nuance - how Russia should pay moral damages - the court's ruling did not specify.

Unfortunately, today it is possible to file a lawsuit against Russia in a Ukrainian court for compensation of damages only with the consent of Russia itself. But such an option may become possible without obtaining the consent of the terrorist state, if appropriate changes are made in the legislation.