That’s up to you! If you want some suggestions there are some ideas below, but don’t be limited by them. Choose two or three themes that are really important to you and that you can write about from experience and with understanding. Pray about your choice of themes.
Your aim is to persuade the bishops to take what you say seriously and to be more accepting and affirming of same-sex relationships. Try to find points in common with each bishop, even if in some cases your views are very different from theirs.
Without telling the bishops what to do, which would be impolite, explain what changes you hope for in the Church’s teaching and practice, why you believe that, and perhaps suggest how those changes could happen.
Although there will probably be some core points that you will want to make to all the bishops (even if in somewhat different words), do remember to bear in mind what you know about each bishop’s views, background etc. It’s not a good idea to write almost the same message to a very conservative anti-gay bishop as to one who is inclusive!