In order to insert blocks of LaTeX notation in your document, click on Insert from the toolbar menu and then click LaTeX. You can choose to set your default to LaTeX by changing your editor settings in your profile.

A new LaTeX block in your document is created where the cursor is. It looks like this:

You can now type some LaTeX syntax. Hovering with your mouse on Preview, you will preview the rendered markdown content. Clicking outside of the block, you will render the content (a latex flag will be added next to the block for you). If your LaTeX contains any errors, we will let you know (an error flag will show next to Preview).

What type of LaTeX is supported?
Authorea supports two engines for rendering LaTeX: LaTeXML and Pandoc. LaTeXML, the most powerful way to render LaTeX notation to the web, supports a large chunk of LaTeX packages and styles. This is an up to date list.

IMPORTANT: How to write LaTeX
DO NOT paste an entire LaTeX article in a LaTeX block! You can import a LaTeX document from your homepage if that is what you intend to do.
Only type LaTeX content in a LaTeX block, i.e. DO NOT include documentclass, preamble, frontmatter, macros or figures. In other words, only include what you would write after \begin{document}.
To add macros (newcommands) and packages, click Settings -> Edit Macros
Use the Insert Figure button to insert images (and data).
Use math mode for equations, e.g. $\mathcal L_{EM}=-\frac14F^{\mu\nu}F_{\mu\nu}$
Try the citation tool (click cite) to find and add citations.
You can use sectioning commands like \section{} and \subsection{} to add headings.