If the introduction is an inverted triangle, the middle section is a sequence of paragraphs that support your thesis, or (in an expository essay) provide the information you promised in your introduction.
The important thing at this stage is to ensure that you construct paragraphs that are unified --one topic per paragraph, each topic suitably and sufficiently supported. If your outline has been carefully thought out, the sequence of paragraphs will make logical sense.
For more, the UVic Writer's Guide has a number of pages on the subject of writing paragraphs paragraphs that you should find helpful.