Building off of the previous post, I would write the letter with envisioning a couple of moments from your own life that could parallel the events about which Miller writes. One such moment could be a time in your life when others have believed gossip over truth, and in the process made judgments that were driven by a lack of evidence. Another could be thinking of what would happen if only "the popular people" ran things. This might be a trivialization, but one of the most haunting aspects of Miller's work is the idea that the legal system and social condemnation was able to be geared towards anyone or anything that was deemed as "different." Think about an experience in your own life- try the school cafeteria- where the popular table controlled all the judgments of others and "held court." This might be an experience that could help create the imagery and personalized sensory imagery that could bring alive the letter of life in Salem in a context that is concrete and meaningful.
This is an assignment that you should write out for yourself, so that it sounds like you, with your own personality and voice. To help you to get started, I would suggest that you pretend to be your same age, and that your mother, aunt, or a friend's mom has been accused of being a witch, and has been taken into the courts and jail. That will give the letter a lot of emotional impact, and you can describe how upsetting it is that she was accused (have her be accused of something silly, like making someone's crops not grow because she sneezed on them once or something), how you know that she is innocent and how you can't believe the courts would condemn her.
Then, describe some of the proceedings in the court. I would have Abigail pretend that she is super cold, and then faint away, along with all of the other girls, when the woman comes into the room. Have the courts use that as evidence for her conviction. Then go on to describe how so many people have been arrested that the farms are going to waste, that cows are roaming the country, and that children are left without their parents. Describe the sadness and chaos that results from all of that.
Since most of the people of Salem were religious people with faith in God, I might talk about that througout your letter--appealing to god for help, hoping he'll save the accused, etc. And, through it all, keep your own voice and personality so that it seems authentic. Does that help to get you started? I hope so; good luck!