I am looking at the letter now, and will summarize it.
First, it’s dated 2 Nitzavim 5591, and addressed to Rabbi Eliyahu Gutmacher, the rav of Pleshen, where the epidemic was happening.
Tefila betzibur should be done in shifts of approximately 15 men, starting at daybreak, and each person should be told what time to come. The same for mincha.
After davening, both morning and night, they should say a few chapters of tehilim, and then Kel Rachum Sh’mecha, Aneinu, Mi She’ana, and the Yehi Ratzon after tehillim, and also pray for the king, his family and ministers, and all the country’s residents. Also at each tefila, morning and night, after Parshas Hatamid the whole Ketores should be said betzibur, and then the Ribon that is printed in the Maamodos for Sunday after the Ketores.
To enforce the 15-man cap ask the police to station a guard. Ask the magistrate and tell him I said so, and if he refuses then write to the government here (in Posen) and you will certainly succeed if you use my name and tell them I gave you all this advice, including to say tehilim and pray for the king.
Also, collect 6 Polish groszy (known in Hebrew as “gedolim”, because of their size) per person, including unborn babies, and make a Pidyon Nefesh with the money, and send some of it to me so I can also make a Pidyon Nefesh for you, and then distribute the money to the poor.
Beware of the cold, each person should wear a flannel cloth over the abdomen. Don’t eat bad things, especially cucumbers. Reduce consumption of fruit, fish, and beer. Don’t eat to satiety, and try to eat many small meals. Be careful with hygiene, don’t let any filth lie around the house, change into clean clothes several times a week. Don’t be depressed, don’t go for walks at night, go out for a walk in the fields on sunny afternoons. Open the windows every morning to air out the rooms. Don’t go out with an empty stomach. Eat a few mustard seeds on an empty stomach. Wash your face and hands every morning with oak bark and water. Clean rooms a few times with strong vinegar mixed with rose water (presumably for the smell).