I want to write a business letter and format an envelope but I cannot find the way to do it
Please, be more elaborative and concrete. What do you mean by ‘format’? What are you trying to do? Place address to specific location, add background image, set envelope size… It’s hard to give an answer if question is vague. This forum is poor training site, but better suited for answering very concrete questions and you probably have more then one (not even realizing it), but every one of them should be asked and answered separately. Vague questions tend to stay unanswered (then closed).
(Did you serach the forum?)
There is Envelope Printing the EASY way e.g. Though it’s an old thread, the location of the ‘Envelope’ dialogue in the menu tree for ‘Writer’ is still unchanged: ‘Insert’ > ‘Envelope…’.
There may also be templates or extensions concerning the topic. Start with this page.