She stays in 1250. She presumably heals a lot of sick people of that era while waiting for another chance to go through the portal.
8) While in 1250, she also leaves clues so that the Eun-soo of 1350 will remember what to do. The writing in her diaries and leaving a message in the film canister.
9) She also has time to figure out time travel dates (maybe not exact times) but at least she knows, from her previous time travel calculations in 1350, what formula and calendar to use.
10) Once she is positive about a date, she prepares herself for another travel through the portal.
11) When she gets to the portal, it is not immediately opened. It takes an unwavering Faith that this time the portal will take her to the right place, only then the portal opens.
12) She goes through it. It brings her back to Seoul. In her voiceover, we hear about this memory, longing and connection that would make her meet him again. Unlike her first time-travel in Ep. 24, this time she doesn’t even glance over modern Seoul because nothing in her heart belongs to 2012.
13) After a mere second in Seoul, she goes back through the portal.
14) This time it takes her to 1355 and there she finds Choi Young.