Twelfth Doctor's regeneration

"Doctor… I let you go."

- The Twelfth Doctor's last words.

Having been gravely wounded in battle against the Cybermen aboard a Mondasian colony ship, (TV: The Doctor Falls) the Twelfth Doctor eventually underwent his regeneration into the Thirteenth Doctor within his TARDIS. (TV: Twice Upon a Time)

Foreshadowing
The Twelfth Doctor had previously faked this event with Nardole’s help. The Twelfth Doctor would test if Bill Potts was under the influence of the Monks by gaslighting her into shooting him. She then did, proving she had free will, the Doctor added the pretend regeneration for a sense of theatre. (TV: The Lie of the Land)

Prolonged beginning
After the Twelfth Doctor was gravely wounded by the Cybermen on the Mondasian colony ship, the regenerative process began. However, tired of "being someone else", the Doctor delayed the change for two weeks (TV: The Doctor Falls) until an encounter with his first incarnation, Archibald Hamish Lethbridge-Stewart and the Testimony caused him to concede that another regeneration would not "kill anyone". After taking a final look at the universe and providing advice to his next incarnation, the Twelfth Doctor regenerated inside his TARDIS in an explosive fashion. (TV: Twice Upon a Time)

The change
As they regenerated, the Doctor relived memories from each of their past lives while the Twelfth Doctor continued to give advice, recalling the magnificence of their TARDIS, (COMIC: The Many Lives of Doctor Who) Ian and Barbara, (COMIC: The Path of Skulls) the strangeness of some of their adventures, (COMIC: Card Conundrum) their love for London (COMIC: Invasion of the Scorpion Men) and for parts of Earth outside the UK, such as New York City. (COMIC: Time Lady of Means) With the new incarnation becoming a certainty instead of a possibility, her actualised potential sent postcards to various friends as a "hello to the world", including V. M. McCrimmon and Grandfather Halfling in the City of the Saved (PROSE: Postscript) and Alistair Gordon Lethbridge-Stewart during his later years. (PROSE: When Times Change...)

As they continued the recollections, the Doctor settled into their new body. (COMIC: The Many Lives of Doctor Who) They next remembered Gallifrey and the regeneration limit, (COMIC: Ophiuchus) how the Sixth Doctor saved the Triumvirs from the Haxeen, (COMIC: Virtually Indestructible) the Master, (COMIC: Crossing the Rubicon) how things were not always as they appeared (COMIC: The Time Ball) and the Last Great Time War. (COMIC: The Whole Thing's Bananas)

As the regeneration finished, the Doctor noticed that her clothes no longer fitted and felt "there was something different about this body". (PROSE: Twice Upon a Time) After the Twelfth Doctor's ring fell off her finger, (TV: Twice Upon a Time) the Doctor remembered how the Ninth Doctor had to relearn to be "the Doctor" after the Time War, (COMIC: Return of the Volsci) how the Tenth Doctor helped Elizabeth Garrett Anderson become the first female doctor in England, (COMIC: Nurse Who?) her wife River Song, (COMIC: Without a Paddle) the Daleks and how they were a constant threat throughout her life, how she lost Bill Potts to the Cybermen, and, finally, how her predecessor was critically injured in the Battle of Floor 0507, which led to the regeneration she was now finishing. (COMIC: Harvest of the Daleks)