We know only a little of what he did.
In "The Night of the Doctor", we learned that, for an unknown length of time, the Eighth Doctor tried to stay out of the War, except for helping its victims when he could. After his encounter with Cass & the spaceship crash on Karn, he decided that he could no longer stay out of the War but had to fight. With the help of one of the Priestesses of Karn, he regenerated into an incarnation who did not use the name "Doctor" (although others continued to call him that). This "War Doctor" incarnation was young-looking at that point.
In "The Day of the Doctor", the War Doctor looked fairly old, implying that considerable time had passed for him since the events of "The Night...", & he was present at the fall of Arcadia (Gallifrey's second city). This event, of which the Tenth Doctor had spoken in "Doomsday" as something he'd not yet "come to terms" with, caused the War Doctor to decide that the Time War had to be ended, no matter what the cost. He therefore stole the Moment, a Time Lord weapon also known as "the Galaxy Eater", & resolved to use it to "destroy Daleks and Time Lords alike" (as the Partisan put it in "The End of Time").
The Moment was unusually powerful & sophisticated (even by Time Lord standards) -- so much so that it had developed a conscience. It persuaded the War Doctor to find an alternative to destroying Gallifrey & brought him the Tenth & Eleventh Doctors, with Clara Oswald, to help him to do so.
Eventually, all thirteen incarnations, including the future Twelfth Doctor, in thirteen instances of the TARDIS, took Gallifrey out of the universe & hid it in a "pocket universe" of its own. The Dalek fleet was surrounding Gallifrey & firing on the planet heavily & continuously when it vanished, so they destroyed themselves almost completely (only a very few survived) & the resulting havoc made it seem as if Gallifrey had also been destroyed.
Because he had crossed his own timestream, the War Doctor & the Tenth Doctor lost their memories of having saved Gallifrey, meaning that the Ninth & Tenth Doctors believed that the War Doctor had used the Moment to end the War by destroying both sides. Only the Eleventh Doctor (from that point in his timestream onwards) -- & Clara -- remembered what had really happened.
- ↑ According to "Engines...", it had been "a hundred years or more" (p20) in the Doctor's timeline since "The Night..." & "Engines..." appears to take place only a short time before "The Day...".