Davros's trial (Revelation of the Daleks)

Following the Dalek-Movellan War, Davros was captured by the Daleks and placed on trial on their home planet, Skaro. Although accounts differed on the exact circumstances, all concurred that Davros was eventually able to turn the tables on the Dalek Empire as a consequence of his trial, rising to the rank of Dalek Emperor himself.

The Time Lords understood Davros' trial, as well as 's trial by the Daleks, to have been conducted for strategic reasons rather than to offer any genuine possibility of clemency. (PROSE: Dalek Combat Training Manual)

Capture
Having been alerted by Takis, a worker at Tranquil Repose on the planet Necros, Daleks loyal to the Supreme Dalek came to Tranquil Repose where they captured Davros. Informing him that he was to stand trial "for crimes against the Daleks", the Daleks took Davros with them. (TV: Revelation of the Daleks)

En route to Skaro, the ship carrying Davros crashed on the planet Lethe. Davros was rescued. He set himself up as "Professor Vaso", altering the perceptions of the humans on the colony so they would not recognise him as the "Great Healer". He attempted to create a new machine, a Juggernaut based on a Mechanoid design.

Lethe's atmosphere prevented the Supreme Dalek retrieving Davros directly, but its forces intercepted the Sixth Doctor's TARDIS, forcing him to serve as an agent of the Daleks and stop Davros' researches and manipulations. The Doctor discovered two of Davros' Necros Daleks had survived the crash, but were destroyed following Davros' final gambit on the colony and the Supreme Dalek's intervention. (AUDIO: The Juggernauts)

Account 1: Before the Golden Emperor
According to one account, Davros was then taken to Skaro, where he was put on trial by the Golden Emperor. He tried to argue that under the Emperor, the Daleks, lacking his guidance, had seen their destiny curtailed, and that only he could allow them to advance once again. Although some Scientist Daleks were privately swayed by the argument, the irate Emperor thundered that it was in command of the Daleks' destiny, not Davros, and ordered his extermination.

Before the sentence could be executed, however, the Sixth Doctor released a computer virus onto the Daleks and saved Davros, taking him on board his TARDIS and telling him of the Hand of Omega, as part of a scheme that would pay off later in both the Daleks' and the Doctor's histories, (COMIC: Emperor of the Daleks!) in the form of the Shoreditch Incident. (TV: Remembrance of the Daleks)

By suggestion of the Doctor, (COMIC: Up Above the Gods) Davros hid himself on Spiridon, along with his empire of Daleks. The Daleks who had put him on trial before came to return him to Skaro, but his Daleks held them off. Davros detonated the planet, killing the Daleks. He woke highly injured, four days into the Imperial-Renegade Dalek Civil War, in a new casing as the Dalek Emperor. (COMIC: Emperor of the Daleks!)

Account 2: Like "second hand goods"
According to a second account, the trial was not a legal endeavor but instead the Daleks "trying out" Davros to solve the schism that had appeared in their empire (AUDIO: Innocence) after the Dalek-Movellan War. (PROSE: Remembrance of the Daleks) As Davros put it, the Daleks were "trying [him] out" as if he were "second hand goods" that needed to be tested before being used. Despite finding the Daleks' request for direction pathetic, he agreed to help and began to reflect on the past. (AUDIO: Innocence)

Account 3: The interference of the Thal
According to another account, Davros encountered the Thal Lareen while en route to Skaro. Lareen was using a stealth suit that made her invisible to Dalek scanners. Lareen attempted to find Davros' "good side," and believed she had succeeded. She gave Davros a capsule containing an enhanced version of the Movellan virus and asked him to release it during his trial. This would destroy all Daleks on Skaro, and make Davros a hero.

Upon arriving on Skaro, Davros found that the Supreme Dalek had a new casing created. With the Golden Emperor apparently absent, the Supreme Dalek planned to be moved to the new casing and named Emperor of the Daleks after Davros' execution. During his trial, Davros told the Daleks of the Movellan virus, and proved his loyalty to the Dalek cause by refusing to release it. He then told them that Lareen was on Skaro and ordered them to exterminate her. The Daleks were impressed with this show of loyalty, and named Davros Emperor of the Daleks. (AUDIO: The Davros Mission)

Behind the scenes
The Discontinuity Guide states that, having been tried by the Supreme Dalek, Davros won the argument regarding human tissue and mentality and took over the white/gold Daleks, augmenting them and himself. He became Emperor, potentially having been influenced by the Fourth Doctor's reference to a Dalek Emperor in the midst of his interrogation during his incursion into Dalek history. However, some Daleks rebelled over the issue of racial purity.