"书信写法" can be translated into English as "the way of writing letters" or more naturally as "letter-writing conventions/practices" or simply "how to write a letter".
"The way of writing letters" directly conveys the method or approach to letter writing.
"Letter-writing conventions/practices" emphasizes the established norms or habits in letter writing.
"How to write a letter" is a more straightforward and practical expression, often used in instructional contexts.
Choose the one that best fits the context you're working with.