I knew greek before, which has substantial overlap.

Flashcards would probably be the fastest method, but I'm not cramming, so to pick up the rest of the characters, I:

- watched the relevant episodes of Спокойной ночи, малыши for letterforms and pronunciation. (phonic segment is about midway through, nearly always at the same time, IIRC)

- read plenty. VK memes at first, news headlines for the proper nouns later.

- watched lyric music videos to work on reading speed.