There is no specific way. 당신 just write to him. Start slow though and don't rush him with things about yourself. And be patient, he will be very hesitant, and doesn't usually write back. He will leave 당신 signs he was there to read it though. Make sure to imagine him opening and 읽기 the letter, it's a universal thing that will send thought waves to him and tell him to come :)
당신 could maybe tell him that 당신 believe in him and 당신 can ask him to visit 또는 say you'll always sing and tell him stories, just some suggestions as they said anything 당신 wanna write really :))
