essay

Hi, I have some questions on how to properly write an essay, it has been a while since the last time a had to write a formal one . so, can I list items, for example...

the symptoms are;
1. one
2. two
3. three

I remember that the linking sentence, goes at the end of the paragraph and introducs the next.

Ex.
not only were they one , but they were also two
Secondly they were two

( I can't think of proper examples lol)

Is this correct, for a formal essay.

Thank you