Question: how was ymir turned into a titan?
Answer: Ymir was able to transform herself into the Jaw Titan (顎の巨人 Agito no Kyojin?), a 5-meter Titan. She gained this ability after she ate Marcel Galliard, a Warrior from Marley, in the year 845. ... This appears to be a defining ability of the Jaw Titan, as Porco Galliard does the same during the Raid on Liberio.