I watched several episodes by myself so let me know once you've caught up.
I watched episodes 10 through 12 by myself already. Let me know when you've caught up with the last episode I watched. Then we can start watching together from episode 13.
ご質問ありがとうございます。
ドラマのエピソードの場合、「数話先に一人で見ちゃったから追いついたら教えて」は英語で「I watched several episodes by myself so let me know once you've caught up.」と言えます。
また、Kyokoさんの英文を英訳しましょう。「episode10-12さきに一人で見ちゃったから、
私が見終えたエピソードまで追いついたら教えて。episode13からまた一緒に見よう」は英語にすると「I watched episodes 10 through 12 by myself already. Let me know when you've caught up with the last episode I watched. Then we can start watching together from episode 13.」という風に言えます。
ご参考になれば幸いです。