Dear XXX, I’m happy to hear about that. Yes, I’d like to accept the offer. Please let me know if I need to take any actions for the next step. Thanks. Sincerely, XXX
일단 사인 하신거 첨부 하시고 “I’m very excited to be on board”, “I look forward to start my career in XXX (회사이름)”, 이런말 적으면 됩니다 별거없어요. 윗분처럼 formal하게 하셔도 되고 살짝 밝게 하셔도 됩니다.